NEET 2010 Biology question

When domestic sewage mixes with river water

(1)The river water is still suitable for drinking as impurities are only about 0.1%
(2)Small animals like rats will die after drinking river water
(3)The increased microbial activity releases micro-nutrients such as iron
(4)The increased microbial activity uses up dissolved oxygenCorrect

Answer: (4)

Solution

Sewage is rich in organic matter. When it mixes with river water, microorganisms multiply and decompose the organic matter, using up the dissolved oxygen, so the biochemical oxygen demand (BOD) rises and the dissolved oxygen falls (fish and other aquatic

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Question ID VK-090430