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Biochemical Oxygen Demand (BOD) in a river water:

1. has no relationship with concentration of oxygen in the water.
2. gives a measure of Salmonella in the water.
3. increases when sewage gets mixed with river water.
4. remains unchanged when algal bloom occurs.

Correct answer: increases when sewage gets mixed with river water.

Solution

Biochemical Oxygen Demand measures the oxygen needed by microorganisms to break down organic matter in water. Sewage adds a large amount of biodegradable material, so the BOD increases.

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