If D.O. concentration falls down to zero in any natural drainage, it indicates the zone of A. Degradation B. Active decomposition C. Recovery D. Cleaner water

The correct answer is A. Degradation.

Dissolved oxygen (DO) is the amount of oxygen that is dissolved in water. It is essential for aquatic life, as it is used in respiration. DO levels can be affected by a number of factors, including temperature, salinity, and pollution.

When DO levels fall below a certain point, it can indicate that the water is polluted. This is because pollution can cause the growth of algae, which use up oxygen as they decompose. As a result, the DO levels in the water can drop to zero, which can kill aquatic life.

The zone of degradation is the area in a natural drainage

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