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. larger number of mosquitoes
B. increase in the amount of dissolved oxygen
C. decrease 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 due to the resulting water pollution, which leads to the death of fish. We need to identify the reason behind the death of fish in such polluted water.

2. **Identifying the Components of Sewage**: Sewage contains organic waste, which is biodegradable. This means that microorganisms can break down these organic materials.

3. **Role of Microorganisms**: When organic waste is introduced into the water, microorganisms such as bacteria begin to decompose this waste. This process is essential for the breakdown of organic matter.

4. **Oxygen Consumption**: The decomposition process requires oxygen. Microorganisms consume dissolved oxygen present in the water to oxidize the organic waste. This leads to a significant reduction in the amount of dissolved oxygen available in the water body.

5. **Biological Oxygen Demand (BOD)**: The amount of oxygen required by microorganisms to decompose organic matter is referred to as Biological Oxygen Demand (BOD). A higher BOD indicates a larger amount of organic waste in the water.

6. **Impact on Fish**: As the microorganisms consume the dissolved oxygen, the concentration of oxygen in the water decreases. Fish and other aquatic organisms rely on dissolved oxygen for survival. A decrease in dissolved oxygen levels can lead to hypoxia (low oxygen conditions), which can be fatal for fish.

7. **Conclusion**: The primary reason for the death of fish in polluted water bodies is the decrease in the amount of dissolved oxygen due to the consumption of oxygen by microorganisms breaking down organic waste.

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

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