Question

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

• A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
• B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
• C. Assertion is correct but Reason is incorrect
• D. Both Assertion and Reason are incorrect

Answer 1

The correct option is A Both Assertion and Reason are correct and Reason is the correct explanation for Assertion

Domestic sewage is rich in biodegradable organic matter and the decomposition of this organic matter by microorganisms requires 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 downstream from the point of sewage discharge. If sewage quantity 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.

So, the correct answer is 'Both Assertion and Reason are correct and Reason is the correct explanation for Assertion'.