BOD and dissolved oxygen link – UGC NET Question

BOD indicates how much oxygen microorganisms need to decompose organic matter. When sewage adds large amounts of biodegradable organic material, microbial activity increases and consumes more dissolved oxygen. As a result, sections of the river with high BOD typically show depressed DO levels, which can stress or kill aquatic organisms. Improvement in water quality reduces BOD and allows DO to recover.

Option A:
Option A is correct as it describes the usual inverse relationship between BOD and DO in organically polluted waters.

Option B:
Option B is incorrect because high BOD and very high DO cannot coexist for long; oxygen is rapidly depleted unless reaeration outpaces consumption.

Option C:
Option C is incorrect since BOD and DO are key linked indicators in water-quality monitoring.

Option D:
Option D is incorrect because when BOD decreases due to reduced organic load, DO normally increases, not declines.