▶️ Answer/Explanation
This question typically refers to the effect of organic pollution (sewage) entering a river. When organic waste is dumped into a river, aerobic bacteria begin to break it down. This process uses up a significant amount of dissolved oxygen, causing the concentration to drop sharply. As the water moves downstream and the waste is fully decomposed, the oxygen levels gradually recover through aeration. Graph $C$ correctly illustrates this “oxygen sag curve,” showing a sudden decrease in oxygen levels at position $Y$ followed by a lower equilibrium or recovery phase.
✅ Answer: (C)
▶️ Answer/Explanation
An acidic environment is defined as having a $pH$ value less than $7.0$. Looking at the provided graph, Species A has a survival range starting exactly at $pH$ $7.0$ and extending to $8.5$. Since no part of its survival bar falls below the neutral point of $7.0$, it cannot survive in acidic conditions. In contrast, Species B, C, and D all have survival bars that extend into the range below $7.0$ (e.g., Species C survives down to $pH$ $4.0$). Therefore, only Species A is strictly limited to neutral and alkaline environments.
✅ Answer: (A)
✅ Answer: (A)
▶️ Answer/Explanation
Pathogens are disease-causing microorganisms, such as bacteria, viruses, and parasites, which are commonly found in human and animal waste. Untreated sewage directly introduces these harmful biological agents into water bodies, leading to severe pollution and health risks. Antibiotics are chemicals that can cause drug resistance but are not primary sources of pathogens. Chlorophyll is a plant pigment and not a pollutant. Methane is a greenhouse gas and does not contain pathogens. Therefore, only untreated sewage is a direct and major source of pathogenic water contamination.
✅ Answer: (D)
✅ Answer: (D)