▶️ Answer/Explanation
Excretion and respiration occur in both animals and plants, since all living organisms must remove metabolic waste and release energy from food.
Movement in plants is usually confined to growth responses (tropisms) rather than whole-organism locomotion, so it is not considered a shared process in the same way.
Row A is the only option in the table that correctly marks excretion and respiration as occurring in both, while movement is not ticked for plants.
✅ Answer: (A)
Movement in plants is usually confined to growth responses (tropisms) rather than whole-organism locomotion, so it is not considered a shared process in the same way.
Row A is the only option in the table that correctly marks excretion and respiration as occurring in both, while movement is not ticked for plants.
✅ Answer: (A)
▶️ Answer/Explanation
Osmosis is defined as the net movement of water molecules across a partially permeable membrane.
Water moves out of cells by osmosis when the external solution has a lower water potential (more concentrated), and into cells when the external solution has a higher water potential (more dilute).
All three statements — the need for a membrane, and movement of water both out of and into cells — are therefore correct.
✅ Answer: (C)
Water moves out of cells by osmosis when the external solution has a lower water potential (more concentrated), and into cells when the external solution has a higher water potential (more dilute).
All three statements — the need for a membrane, and movement of water both out of and into cells — are therefore correct.
✅ Answer: (C)
▶️ Answer/Explanation
Carbohydrates and fats are made only of carbon, hydrogen and oxygen.
Proteins are made of carbon, hydrogen, oxygen, and additionally nitrogen (found in the amino group of every amino acid).
Nitrogen is therefore the element present in all proteins but not present in carbohydrates or fats.
✅ Answer: (D)
Proteins are made of carbon, hydrogen, oxygen, and additionally nitrogen (found in the amino group of every amino acid).
Nitrogen is therefore the element present in all proteins but not present in carbohydrates or fats.
✅ Answer: (D)
▶️ Answer/Explanation
Enzymes have an optimum temperature at which their rate of reaction is highest, and this optimum reflects the normal environment of the organism.
Since the Pompeii worm lives at temperatures around 68°C, its enzymes must have an unusually high optimum temperature compared with typical organisms.
Graph B shows the rate of reaction peaking at a high temperature (close to 68°C) before denaturation causes a sharp fall, matching this worm’s enzymes.
✅ Answer: (B)
Since the Pompeii worm lives at temperatures around 68°C, its enzymes must have an unusually high optimum temperature compared with typical organisms.
Graph B shows the rate of reaction peaking at a high temperature (close to 68°C) before denaturation causes a sharp fall, matching this worm’s enzymes.
✅ Answer: (B)
▶️ Answer/Explanation
Chlorophyll is the green pigment in chloroplasts that absorbs light energy for photosynthesis.
Magnesium is a structural component of the chlorophyll molecule itself, so a deficiency of magnesium ions leads to yellowing of leaves (chlorosis).
Nitrate is instead needed for making amino acids and proteins, and iron/calcium are not part of the chlorophyll structure.
✅ Answer: (C)
Magnesium is a structural component of the chlorophyll molecule itself, so a deficiency of magnesium ions leads to yellowing of leaves (chlorosis).
Nitrate is instead needed for making amino acids and proteins, and iron/calcium are not part of the chlorophyll structure.
✅ Answer: (C)
▶️ Answer/Explanation
Ingestion is taking food into the mouth, absorption is movement of digested food into the blood, and egestion is passing out undigested food as faeces.
Assimilation is the final stage, where absorbed nutrients move into body cells and are used, becoming part of those cells (e.g. amino acids built into new proteins).
Option A describes exactly this process, distinguishing it from simple absorption in option B.
✅ Answer: (A)
Assimilation is the final stage, where absorbed nutrients move into body cells and are used, becoming part of those cells (e.g. amino acids built into new proteins).
Option A describes exactly this process, distinguishing it from simple absorption in option B.
✅ Answer: (A)
▶️ Answer/Explanation
Higher temperatures increase the rate of evaporation of water from the leaf’s mesophyll cells and increase the kinetic energy of water vapour molecules, speeding up diffusion out of the stomata.
Low humidity means the air outside the leaf holds less water vapour, creating a steeper concentration gradient for diffusion from the leaf’s air spaces to the atmosphere.
Combining high temperature with low humidity therefore produces the greatest possible transpiration rate.
✅ Answer: (D)
Low humidity means the air outside the leaf holds less water vapour, creating a steeper concentration gradient for diffusion from the leaf’s air spaces to the atmosphere.
Combining high temperature with low humidity therefore produces the greatest possible transpiration rate.
✅ Answer: (D)
▶️ Answer/Explanation
Air travels in order from the larynx (voice box, top of the airway) down into the trachea (windpipe), which splits into two bronchi, which further divide into narrower bronchioles.
Matching this order onto the diagram’s labelled structures S, P, R and Q gives the correct sequence: larynx = S, bronchus = P, trachea = R, bronchioles = Q.
✅ Answer: (C)
Matching this order onto the diagram’s labelled structures S, P, R and Q gives the correct sequence: larynx = S, bronchus = P, trachea = R, bronchioles = Q.
✅ Answer: (C)
▶️ Answer/Explanation
In cold conditions, blood vessels near the skin surface constrict (vasoconstriction) to reduce heat loss by directing blood flow deeper into the body.
Hair erector muscles also contract, pulling hairs upright to trap an insulating layer of air near the skin surface.
The two labelled structures corresponding to a constricted blood vessel and an erected hair are the ones that help retain heat in the cold.
✅ Answer: (A)
Hair erector muscles also contract, pulling hairs upright to trap an insulating layer of air near the skin surface.
The two labelled structures corresponding to a constricted blood vessel and an erected hair are the ones that help retain heat in the cold.
✅ Answer: (A)
▶️ Answer/Explanation
An egg is released from the ovary into the oviduct (fallopian tube), where fertilisation would normally occur.
If unfertilised, it continues into the uterus, then passes through the narrow cervix, and finally leaves the body via the vagina.
This gives the order oviduct → uterus → cervix → vagina.
✅ Answer: (B)
If unfertilised, it continues into the uterus, then passes through the narrow cervix, and finally leaves the body via the vagina.
This gives the order oviduct → uterus → cervix → vagina.
✅ Answer: (B)
▶️ Answer/Explanation
An allele is defined as an alternative version of a gene, arising from a difference in DNA base sequence at the same gene location.
DNA is present in almost all body cells (not only gametes), genes code for proteins (not fats directly), and human males have one X and one Y chromosome, not two X chromosomes.
Only the statement about alleles being versions of genes is scientifically correct.
✅ Answer: (A)
DNA is present in almost all body cells (not only gametes), genes code for proteins (not fats directly), and human males have one X and one Y chromosome, not two X chromosomes.
Only the statement about alleles being versions of genes is scientifically correct.
✅ Answer: (A)
▶️ Answer/Explanation
The beech tree is a producer (makes its own food by photosynthesis), and the insect is a primary consumer/herbivore, not a producer.
The shrew eats insects, making it a carnivore rather than a herbivore, and the owl eats the shrew, so the owl is also a carnivore.
Since the owl feeds on another animal (the shrew), it is correctly classified as a carnivore.
✅ Answer: (C)
The shrew eats insects, making it a carnivore rather than a herbivore, and the owl eats the shrew, so the owl is also a carnivore.
Since the owl feeds on another animal (the shrew), it is correctly classified as a carnivore.
✅ Answer: (C)
▶️ Answer/Explanation
Carbon dioxide from the atmosphere enters plants through photosynthesis (process X), building carbon compounds in plant tissue.
Carbon then passes from plants to animals through feeding (one of the Y arrows), while respiration in both plants and animals releases carbon dioxide back to the atmosphere (the other Y arrow).
This matches X = photosynthesis, Y = respiration, Y = feeding.
✅ Answer: (B)
Carbon then passes from plants to animals through feeding (one of the Y arrows), while respiration in both plants and animals releases carbon dioxide back to the atmosphere (the other Y arrow).
This matches X = photosynthesis, Y = respiration, Y = feeding.
✅ Answer: (B)
▶️ Answer/Explanation
A mixture (P) can be separated using physical methods such as filtration or distillation, since its components are not chemically bonded.
A compound (Q) can only be broken down into simpler substances by chemical reactions, as its atoms are chemically bonded together.
An element (R) cannot be broken down further by either physical or chemical means, since it consists of only one type of atom.
✅ Answer: (D)
A compound (Q) can only be broken down into simpler substances by chemical reactions, as its atoms are chemically bonded together.
An element (R) cannot be broken down further by either physical or chemical means, since it consists of only one type of atom.
✅ Answer: (D)
▶️ Answer/Explanation
Balancing carbon first: \( C_3H_6 \) has 3 carbons, so \( y = 3 \) for \( CO_2 \); balancing hydrogen: 6 hydrogens need \( z = 3 \) for \( H_2O \) (since each \( H_2O \) has 2 H).
Balancing oxygen: the right side has \( (3\times2) + (3\times1) = 9 \) oxygen atoms, requiring \( x = 4.5 \) if \( w=1 \); doubling everything to avoid fractions gives \( w=2, x=9, y=6, z=6 \).
This gives the fully balanced equation \( 2C_3H_6 + 9O_2 \rightarrow 6CO_2 + 6H_2O \).
✅ Answer: (C)
Balancing oxygen: the right side has \( (3\times2) + (3\times1) = 9 \) oxygen atoms, requiring \( x = 4.5 \) if \( w=1 \); doubling everything to avoid fractions gives \( w=2, x=9, y=6, z=6 \).
This gives the fully balanced equation \( 2C_3H_6 + 9O_2 \rightarrow 6CO_2 + 6H_2O \).
✅ Answer: (C)
▶️ Answer/Explanation
A gas that ignites with a ‘pop’ when tested with a lighted splint is the standard test result for hydrogen gas.
During electrolysis, positively charged hydrogen ions (\(H^+\)) are attracted to the negative electrode (cathode), where they gain electrons and are reduced to hydrogen gas.
Gas G is therefore hydrogen, formed at the cathode.
✅ Answer: (B)
During electrolysis, positively charged hydrogen ions (\(H^+\)) are attracted to the negative electrode (cathode), where they gain electrons and are reduced to hydrogen gas.
Gas G is therefore hydrogen, formed at the cathode.
✅ Answer: (B)
▶️ Answer/Explanation
A decrease in temperature during a process shows that thermal energy has been taken in from the surroundings rather than released.
Reactions or processes that absorb thermal energy from the surroundings, lowering the temperature, are classified as endothermic.
Since dissolving ammonium nitrate here causes the water to cool, this is an endothermic process, not exothermic, oxidation or reduction.
✅ Answer: (A)
Reactions or processes that absorb thermal energy from the surroundings, lowering the temperature, are classified as endothermic.
Since dissolving ammonium nitrate here causes the water to cool, this is an endothermic process, not exothermic, oxidation or reduction.
✅ Answer: (A)
▶️ Answer/Explanation
Options A, C and D all involve a substance gaining or losing oxygen (reduction and oxidation happening together), so they are redox reactions.
The reaction between silver nitrate and sodium chloride is a precipitation (double decomposition) reaction, where ions simply swap partners to form insoluble silver chloride, with no change in oxidation state.
No oxygen transfer or electron transfer occurs in this reaction, so it is not a redox reaction.
✅ Answer: (B)
The reaction between silver nitrate and sodium chloride is a precipitation (double decomposition) reaction, where ions simply swap partners to form insoluble silver chloride, with no change in oxidation state.
No oxygen transfer or electron transfer occurs in this reaction, so it is not a redox reaction.
✅ Answer: (B)
▶️ Answer/Explanation
Adding acidified silver nitrate solution to a halide solution produces a precipitate whose colour identifies the specific halide present.
Chloride ions give a white precipitate, bromide ions give a cream precipitate, and iodide ions give a yellow precipitate.
Option C describes the test for ammonium ions, and option D describes the test for a carbonate, so the correct test and result for bromide is option A.
✅ Answer: (A)
Chloride ions give a white precipitate, bromide ions give a cream precipitate, and iodide ions give a yellow precipitate.
Option C describes the test for ammonium ions, and option D describes the test for a carbonate, so the correct test and result for bromide is option A.
✅ Answer: (A)
▶️ Answer/Explanation
Going down Group I, the outer electron becomes further from the nucleus and more shielded, so it is lost more easily, making the elements more reactive.
At the same time, the metallic bonding becomes weaker as the atoms get larger, so less energy is needed to melt the metal, and melting point decreases down the group.
This gives increasing reactivity and decreasing melting point.
✅ Answer: (C)
At the same time, the metallic bonding becomes weaker as the atoms get larger, so less energy is needed to melt the metal, and melting point decreases down the group.
This gives increasing reactivity and decreasing melting point.
✅ Answer: (C)
▶️ Answer/Explanation
Helium is a noble gas, meaning it is chemically unreactive (inert) due to its full outer electron shell.
It is also much less dense than air, allowing airships filled with it to float, and unlike hydrogen it is non-flammable, making it much safer to use.
Carbon dioxide and nitrogen are both denser than helium and would not provide the same lift.
✅ Answer: (B)
It is also much less dense than air, allowing airships filled with it to float, and unlike hydrogen it is non-flammable, making it much safer to use.
Carbon dioxide and nitrogen are both denser than helium and would not provide the same lift.
✅ Answer: (B)
▶️ Answer/Explanation
Metals and alloys are giant structures of atoms, not molecules, so alloys are correctly described as being made up of atoms of each element.
In a pure metal, atoms are all the same size and can slide over one another easily, but alloying introduces atoms of a different size that disrupt this regular arrangement.
This disruption prevents layers of atoms from sliding smoothly, making the alloy harder than the pure metal.
✅ Answer: (A)
In a pure metal, atoms are all the same size and can slide over one another easily, but alloying introduces atoms of a different size that disrupt this regular arrangement.
This disruption prevents layers of atoms from sliding smoothly, making the alloy harder than the pure metal.
✅ Answer: (A)
▶️ Answer/Explanation
Metals less reactive than carbon can be extracted from their ores by heating (reduction) with carbon, since carbon can displace them from their oxides.
Copper is below carbon in the reactivity series, so it can be extracted this way, whereas magnesium, sodium and potassium are all more reactive than carbon.
Metals more reactive than carbon must instead be extracted by electrolysis.
✅ Answer: (A)
Copper is below carbon in the reactivity series, so it can be extracted this way, whereas magnesium, sodium and potassium are all more reactive than carbon.
Metals more reactive than carbon must instead be extracted by electrolysis.
✅ Answer: (A)
▶️ Answer/Explanation
Anhydrous cobalt(II) chloride is blue in its dry, water-free form.
When water is added, it absorbs water molecules and turns pink, forming hydrated cobalt(II) chloride.
This blue-to-pink colour change is the standard chemical test for the presence of water.
✅ Answer: (A)
When water is added, it absorbs water molecules and turns pink, forming hydrated cobalt(II) chloride.
This blue-to-pink colour change is the standard chemical test for the presence of water.
✅ Answer: (A)
▶️ Answer/Explanation
Lime (calcium oxide) is a metal oxide and calcium hydroxide is a metal hydroxide, both of which are bases that react with and neutralise acids.
Limestone (calcium carbonate) is a metal carbonate, and carbonates also react with and neutralise acids, releasing carbon dioxide gas in the process.
All three substances are therefore capable of neutralising acids.
✅ Answer: (D)
Limestone (calcium carbonate) is a metal carbonate, and carbonates also react with and neutralise acids, releasing carbon dioxide gas in the process.
All three substances are therefore capable of neutralising acids.
✅ Answer: (D)
▶️ Answer/Explanation
Complete combustion occurs when a hydrocarbon burns in a plentiful (excess) supply of oxygen.
Under these conditions, all the carbon in the fuel is fully oxidised to carbon dioxide, and all the hydrogen is oxidised to water, with no carbon monoxide or soot (carbon) produced.
Incomplete combustion, which produces carbon monoxide or carbon, only occurs with a limited oxygen supply.
✅ Answer: (C)
Under these conditions, all the carbon in the fuel is fully oxidised to carbon dioxide, and all the hydrogen is oxidised to water, with no carbon monoxide or soot (carbon) produced.
Incomplete combustion, which produces carbon monoxide or carbon, only occurs with a limited oxygen supply.
✅ Answer: (C)
▶️ Answer/Explanation
Poly(ethene) is a polymer, not an alkene itself — it is made from many ethene (an alkene) monomer molecules joining together.
Cracking is the process that produces smaller alkenes like ethene from larger hydrocarbons, while poly(ethene) is instead produced from ethene by an addition reaction, with monomers joining without any small molecule being lost.
It is not obtained from the bitumen fraction, since that fraction is used for surfacing roads.
✅ Answer: (D)
Cracking is the process that produces smaller alkenes like ethene from larger hydrocarbons, while poly(ethene) is instead produced from ethene by an addition reaction, with monomers joining without any small molecule being lost.
It is not obtained from the bitumen fraction, since that fraction is used for surfacing roads.
✅ Answer: (D)
▶️ Answer/Explanation
Density is defined as the mass of a substance per unit volume it occupies.
This is written as \( \text{density} = \dfrac{\text{mass}}{\text{volume}} \), with units such as \( g/cm^3 \) or \( kg/m^3 \).
None of the other expressions correctly represent this physical quantity.
✅ Answer: (A)
This is written as \( \text{density} = \dfrac{\text{mass}}{\text{volume}} \), with units such as \( g/cm^3 \) or \( kg/m^3 \).
None of the other expressions correctly represent this physical quantity.
✅ Answer: (A)
▶️ Answer/Explanation
The moment of a force is calculated using \( \text{moment} = \text{force} \times \text{perpendicular distance from the pivot} \).
The perpendicular distance between the 35 cm mark (where the force acts) and the 60 cm pivot is \( 60 – 35 = 25\text{ cm} \).
So the moment is \( 6.0\text{ N} \times 25\text{ cm} = 150\text{ Ncm} \).
✅ Answer: (A)
The perpendicular distance between the 35 cm mark (where the force acts) and the 60 cm pivot is \( 60 – 35 = 25\text{ cm} \).
So the moment is \( 6.0\text{ N} \times 25\text{ cm} = 150\text{ Ncm} \).
✅ Answer: (A)
▶️ Answer/Explanation
Work done is calculated as \( \text{work} = \text{force} \times \text{distance moved} \), where the force needed equals the weight of the load.
Lifting the load higher increases distance (more work), and the speed of lifting or the power of the crane affects only how quickly work is done, not the total amount of work.
Reducing the mass of the load reduces its weight, and therefore reduces the force needed, decreasing the total work done.
✅ Answer: (C)
Lifting the load higher increases distance (more work), and the speed of lifting or the power of the crane affects only how quickly work is done, not the total amount of work.
Reducing the mass of the load reduces its weight, and therefore reduces the force needed, decreasing the total work done.
✅ Answer: (C)
▶️ Answer/Explanation
Hydroelectric, tidal and wave power are all renewable resources, since they rely on natural processes (water flow, tides, waves) that are continuously replenished.
Nuclear fission instead relies on a finite supply of uranium fuel that is mined from the Earth and cannot be replenished once used.
This makes nuclear fission the only non-renewable energy source in the list.
✅ Answer: (B)
Nuclear fission instead relies on a finite supply of uranium fuel that is mined from the Earth and cannot be replenished once used.
This makes nuclear fission the only non-renewable energy source in the list.
✅ Answer: (B)
▶️ Answer/Explanation
The particle diagram shows a change from a more randomly arranged, freely-moving state to a tightly packed, regularly-ordered state.
This ordered, closely-packed arrangement with particles only vibrating in fixed positions is characteristic of a solid.
A liquid changing into this solid arrangement is the process of solidification (freezing).
✅ Answer: (D)
This ordered, closely-packed arrangement with particles only vibrating in fixed positions is characteristic of a solid.
A liquid changing into this solid arrangement is the process of solidification (freezing).
✅ Answer: (D)
▶️ Answer/Explanation
Thermal energy travels through solids by conduction, since convection requires a fluid (liquid or gas) that can circulate, which is not possible in a solid object.
Iron, as a metal, is a good thermal conductor due to its free (delocalised) electrons that transfer energy quickly, whereas wood is a poor conductor (a good insulator).
Most thermal energy therefore reaches point P by conduction through the iron rather than through the wood.
✅ Answer: (A)
Iron, as a metal, is a good thermal conductor due to its free (delocalised) electrons that transfer energy quickly, whereas wood is a poor conductor (a good insulator).
Most thermal energy therefore reaches point P by conduction through the iron rather than through the wood.
✅ Answer: (A)
▶️ Answer/Explanation
Frequency is the number of wavefronts (crests) produced per second, so \( f = \dfrac{1}{0.50\text{ s}} = 2.0\text{ Hz} \).
The diagram shows 6.0 cm spanning two full wavelengths, so one wavelength is \( \dfrac{6.0}{2} = 3.0\text{ cm} \).
This gives a frequency of 2.0 Hz and a wavelength of 3.0 cm.
✅ Answer: (C)
The diagram shows 6.0 cm spanning two full wavelengths, so one wavelength is \( \dfrac{6.0}{2} = 3.0\text{ cm} \).
This gives a frequency of 2.0 Hz and a wavelength of 3.0 cm.
✅ Answer: (C)
▶️ Answer/Explanation
Angles of incidence and reflection are always measured from the normal, a line perpendicular to the mirror surface at the point of reflection, not from the mirror surface itself.
The 60° marked in the diagram is measured from the normal, giving both the angle of incidence and the angle of reflection as 60°, in line with the law of reflection.
The 30° shown is the angle between the ray and the mirror surface, not the angle used in the law of reflection.
✅ Answer: (D)
The 60° marked in the diagram is measured from the normal, giving both the angle of incidence and the angle of reflection as 60°, in line with the law of reflection.
The 30° shown is the angle between the ray and the mirror surface, not the angle used in the law of reflection.
✅ Answer: (D)
▶️ Answer/Explanation
Resistance is calculated using \( R = \dfrac{V}{I} \), where V is the voltage across the component and I is the current through it.
Substituting the given values gives \( R = \dfrac{6.0\text{ V}}{0.60\text{ A}} = 10\ \Omega \).
This is the resistance of the lamp in this circuit.
✅ Answer: (C)
Substituting the given values gives \( R = \dfrac{6.0\text{ V}}{0.60\text{ A}} = 10\ \Omega \).
This is the resistance of the lamp in this circuit.
✅ Answer: (C)
▶️ Answer/Explanation
In a series circuit, current has only one path to flow through, so the current (and therefore the ammeter reading) is the same at every point in the circuit.
The series circuit’s total resistance is actually \( 3.0 + 3.0 = 6.0\ \Omega \), and the parallel circuit’s total resistance is \( 1.5\ \Omega \) (less than either resistor), so options A and B are incorrect.
In the parallel circuit, ammeter P (measuring the total current from the supply) reads the sum of the two branch currents, so it reads more, not less, than the branch ammeters.
✅ Answer: (C)
The series circuit’s total resistance is actually \( 3.0 + 3.0 = 6.0\ \Omega \), and the parallel circuit’s total resistance is \( 1.5\ \Omega \) (less than either resistor), so options A and B are incorrect.
In the parallel circuit, ammeter P (measuring the total current from the supply) reads the sum of the two branch currents, so it reads more, not less, than the branch ammeters.
✅ Answer: (C)
▶️ Answer/Explanation
A fuse is meant to blow (melt) and break the circuit before the current gets high enough to damage the cable, since the cable is only rated to safely carry up to 6 A.
The original 5 A fuse blew because the current drawn already exceeded a safe level, so replacing it with a 13 A fuse allows a current well above the cable’s 6 A safe rating to flow without the fuse breaking the circuit.
This means the cable itself could overheat and potentially cause a fire before the oversized fuse ever blows.
✅ Answer: (B)
The original 5 A fuse blew because the current drawn already exceeded a safe level, so replacing it with a 13 A fuse allows a current well above the cable’s 6 A safe rating to flow without the fuse breaking the circuit.
This means the cable itself could overheat and potentially cause a fire before the oversized fuse ever blows.
✅ Answer: (B)
▶️ Answer/Explanation
Alpha particles are large and heavily charged, so they interact strongly with atoms they pass, making them highly ionising, but this also means they lose their energy very quickly.
Because of this rapid energy loss, alpha particles can only travel a short distance and are stopped easily (e.g. by paper or skin), making them far less penetrating than beta particles.
Beta particles are smaller and less charged, so they ionise less per collision but travel further, making them more penetrating than alpha.
✅ Answer: (B)
Because of this rapid energy loss, alpha particles can only travel a short distance and are stopped easily (e.g. by paper or skin), making them far less penetrating than beta particles.
Beta particles are smaller and less charged, so they ionise less per collision but travel further, making them more penetrating than alpha.
✅ Answer: (B)
▶️ Answer/Explanation
Each half-life reduces the number of remaining atoms by half: 80 million → 40 million (1 half-life) → 20 million (2 half-lives) → 10 million (3 half-lives).
Three half-lives have elapsed to reach 10 million atoms from the original 80 million.
Total time = \( 3 \times 18\text{ years} = 54\text{ years} \).
✅ Answer: (C)
Three half-lives have elapsed to reach 10 million atoms from the original 80 million.
Total time = \( 3 \times 18\text{ years} = 54\text{ years} \).
✅ Answer: (C)