Assertion : There is a sharp decline in dissolved oxygen downstream from the point of sewage discharge.
Reason : Microoganisms ionvolved in biodegradation or organic matter in the receiving water body consume a lot of oxygen.

Answer

A

Explanation

Domestic sewage is rich in biodegradable organic matter and the decomposition of this organic matter by microorganisms require oxygen. Microorganisms involved in biodegradation of organic matter in the receiving water body consume a lot of oxygen, and as a result there is a sharp decline in dissolved oxygen, and as a result there is a sharp decline in dissolved oxygen downstream from the point of sewage discharge. If sewage quantiy is large, the whole of dissolved oxygen may be consumed leaving nothing for respiration for fish and other clean water organisms. They, therefore, get killed. However, as sewage is decomposed, there is a gradual rise in dissolved oxygen downstream. Fish and other clean water organisms reappear indicating the recovery of river from sewage pollution.