Home Biology Ecosystem Previous Year Questions When domestic sewage mixes with river water …
Biology Ecosystem Previous Year Questions Single Correct MCQ
Published on: August 12, 2026

When domestic sewage mixes with river water :-

A
The increased microbial activity releases micro-nutrients such as iron.
B
The increased microbial activity uses up dissolved oxygen.
C
The river water is still suitable for drinking as impurities are only about 0.1%
D
Small animals like rats will die after drinking river water.

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Text Solution

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The correct answer is:
B

When domestic sewage enters river water, it adds a large amount of organic matter. Microorganisms decompose this waste, and during decomposition they consume dissolved oxygen (DO), which can harm aquatic life.

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