Sewage containing organic waste should not be disposed in water bodies because it causes major water pollution. Fishes in such a polluted water die because of

Options

A. large number of mosquitoes
B. increase in the amount of dissolved oxygen
C. decrese in the amount of dissolved oxygen in water
D. clogging of gills by mud

Explanation

### Step-by-Step Solution:

1. **Understanding the Problem**: The question states that sewage containing organic waste should not be disposed of in water bodies because it causes major water pollution. We need to explain why fish die in such polluted water.

2. **Identifying the Components of Sewage**: Sewage contains organic waste, which is biodegradable material that can decompose.

3. **Oxygen Demand**: When organic waste is introduced into water bodies, it requires oxygen to decompose. This oxygen is taken from the dissolved oxygen present in the water.

4. **Biochemical Oxygen Demand (BOD)**: The amount of oxygen required by microorganisms to decompose organic matter in water is referred to as Biochemical Oxygen Demand (BOD). When organic waste is added to water, the BOD increases.

5. **Effect on Dissolved Oxygen**: As the BOD increases due to the presence of organic waste, the dissolved oxygen (O2) in the water decreases. This is because the microorganisms consume the dissolved oxygen to break down the organic matter.

6. **Impact on Aquatic Life**: Fish and other aquatic organisms rely on dissolved oxygen for respiration. When the levels of dissolved oxygen drop significantly, fish find it difficult to breathe, leading to their death.

7. **Conclusion**: Therefore, the correct answer to why fish die in polluted water is due to the **decrease in the amount of dissolved oxygen**.

### Final Answer:
Fish in such polluted water die because of the **decrease in the amount of dissolved oxygen**.

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