Q. When domestic sewage mixes with river water
A) Small animals like rats will die after drinking river water
B) The increased microbial activity releases micro-nutrients such as iron
C) The increased microbial activity uses up dissolved oxygen
D) The river water is still suitable for drinking as impurities are only about 0.1%
Explanation
[Solution from source] Sewage adds a large amount of biodegradable organic matter, and the microbes decomposing it consume dissolved oxygen, raising the BOD and causing oxygen depletion that kills fish. Even though impurities are a small percentage, they include pathogens, so the water is not fit for drinking. Answer (3).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.