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Assertion : BOD of a river polluted by sewage is more than 20 ppm.

Reason : Polluted river contains excess of organic matter.

(a) Both A and R are true and R is the correct explanation of A

(b) Both A and R are true but R is not correct explanation of A

(c) A is true but R is false

(d) A and R are false

1 Answer

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Best answer

Correct option (a) 

Explanation :

Degree of impurity of water due to organic matter is measured in terms of BOD (Biochemical oxygen demand). BOD is the oxygen in milligrams required for five days in one litre of water at 20°C for the microorganisms to metabolise organic waste. It is measured in the milligrams of oxygen taken up in one litre of sewage. The effluent that is legally allowed to be discharged into a river or stream should be such that BOD at 20°C should not be more than 20 ppm, i.e., 20 mg per litre.

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