A decreasing dissolved oxygen (DO) level in a water body:

1.Severely threatens aquatic life.

2.Is mainly caused by oxygen-demanding wastes.

3.Indicates the presence of biodegradable organic substances.

4.Causes undesired taste and colour.

5.Makes it non-drinkable to human beings.

A1, 2, 3 & 4 only ✓ Correct

B2, 3, 4 & 5 only

C1, 3, 4 & 5 only

D1, 2, 4 & 5 only

Correct answer: (A) 1, 2, 3 & 4 only

Explanation

Falling dissolved oxygen threatens aquatic life, stems from oxygen-demanding wastes, signals biodegradable matter and spoils taste and colour.

Statement 1 is correct, as fish and other aquatic organisms suffocate when oxygen drops.

Statement 2 is correct, since organic wastes consume oxygen as they decompose.

Statement 3 is correct, as low oxygen reveals biodegradable organic loading.

Statement 4 is correct, since resulting anaerobic conditions cause bad taste and colour.

Statement 5 is wrong, as dissolved oxygen level alone does not decide drinkability.

Oxygen-demanding wastes are measured through biochemical oxygen demand.

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