Assertion : BOD of river polluted by sewage is more than 20 ppm.
Reason: Polluted river contains excess of organic matter.

Options

A. If both are true with reason being correct explanation
B. Both true but reason is not correct explanation
C. assertion is true but reason is wrong
D. Both are wrong

Answer

### Step-by-Step Solution:

1. **Understanding BOD**:
- BOD stands for Biochemical Oxygen Demand. It measures the amount of oxygen that microorganisms will consume while decomposing organic matter in water over a specific period (usually 5 days) at a specific temperature (20°C).
- The higher the BOD, the more polluted the water is, as it indicates a higher level of organic matter present.

2. **Analyzing the Assertion**:
- The assertion states that the BOD of a river polluted by sewage is more than 20 ppm (parts per million).
- According to environmental standards, a BOD level above 20 ppm indicates significant pollution due to organic waste, which is consistent with sewage contamination.

3. **Analyzing the Reason**:
- The reason states that a polluted river contains an excess of organic matter.
- This is true because sewage introduces a large amount of organic material into the water, which microorganisms break down, leading to increased BOD levels.

4. **Evaluating the Relationship**:
- Both the assertion and the reason are correct statements.
- The reason (excess organic matter) explains why the BOD is high (more than 20 ppm) in the polluted river.

5. **Conclusion**:
- Since both the assertion and the reason are true, and the reason justifies the assertion, we conclude that the correct answer is that both statements are true and the reason is the correct explanation of the assertion.

### Final Answer:
Both the assertion and the reason are true, and the reason correctly explains the assertion.

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