Sewage drained into water bodies kill fishes because

Options

A. excessive carbon dioxide is added to water
B. it gives off a bad smell
C. it removes the food eaten by fish
D. it increases competition with fishes for dissolved oxygen

Explanation

### Step-by-Step Text Solution:

1. **Understanding Sewage and Water Bodies**: Sewage contains organic pollutants that are harmful to aquatic life. When sewage is drained into water bodies, it introduces these pollutants into the ecosystem.

2. **Nutrient Enrichment**: The organic pollutants in sewage act as nutrients for algae. This leads to a rapid increase in algal growth, known as an algal bloom.

3. **Impact of Algal Bloom**: The excessive growth of algae can block sunlight from reaching submerged aquatic plants. This lack of light prevents photosynthesis in these plants, leading to their death.

4. **Decomposition of Algae and Plants**: When the algae and the submerged plants die, they undergo decomposition. This process is carried out by bacteria, which consume oxygen from the water.

5. **Oxygen Depletion**: The decomposition of dead algae and plants significantly reduces the dissolved oxygen levels in the water. This phenomenon is known as oxygen depletion.

6. **Competition for Oxygen**: Fish and other aquatic organisms rely on dissolved oxygen for survival. As the oxygen levels decrease due to the decomposition process, the competition for the limited oxygen increases among the fish and other aquatic organisms.

7. **Conclusion**: The overall effect of sewage draining into water bodies is that it leads to increased competition for dissolved oxygen among fish, ultimately resulting in their death.

### Final Answer:
Sewage drained into water bodies kills fishes because it increases the competition for dissolved oxygen due to algal blooms and subsequent oxygen depletion.

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