The algae dies out in the zone of A. Degradation B. Active decomposition C. Recovery D. Cleaner water

The correct answer is: A. Degradation.

Algae dies out in the zone of degradation because the water is too polluted. The zone of degradation is the area of a lake or river where the water is too polluted for algae to survive. The water in this zone is usually very dark and has a lot of organic matter. This organic matter provides food for bacteria, which break it down and release nutrients into the water. These nutrients stimulate the growth of algae, which eventually die and decompose. The decomposition of algae uses up oxygen in the water, which can suffocate fish and other aquatic life.

The other options are incorrect because:

• Option B, active decomposition, is the area of a lake or river where the algae is breaking down. This is not the area where the algae dies out.
• Option C, recovery, is the area of a lake or river where the algae is starting to grow again. This is not the area where the algae dies out.
• Option D, cleaner water, is the area of a lake or river where the water is not polluted. This is not the area where the algae dies out.