Assertion (A): Blue green algae are popular as biofertilisers. Reason (R) : Blue green algae cause algal bloom in polluted water bodies.

Options

A. Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
B. Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).
C. Assertion (A) is true but reason (R) is false.
D. Assertion (A) is false but reason (R) is true.

Explanation

To analyze the assertion and reason provided in the question, we can break down the steps as follows:

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that "Blue green algae are popular as biofertilisers."
- We need to evaluate whether blue green algae are indeed considered popular biofertilisers.

2. **Evaluating the Assertion**:
- Blue green algae, also known as cyanobacteria, can contribute to soil fertility by fixing atmospheric nitrogen.
- However, they are not widely recognized or used as biofertilisers in agricultural practices compared to other organisms like rhizobia or mycorrhizae.
- Therefore, the assertion is **incorrect**.

3. **Understanding the Reason (R)**:
- The reason states that "Blue green algae cause algal bloom in polluted water bodies."
- Algal blooms are indeed a phenomenon where excessive growth of algae occurs, often due to nutrient pollution (like nitrogen and phosphorus) in water bodies.

4. **Evaluating the Reason**:
- Blue green algae can thrive in nutrient-rich waters and often lead to algal blooms, which can be harmful to aquatic ecosystems.
- Hence, the reason is **correct**.

5. **Conclusion**:
- Since the assertion is incorrect and the reason is correct, we can conclude that the correct option is that the assertion is wrong but the reason is true.

### Final Answer:
- The correct option is **D**: Assertion is wrong, but Reason is true.

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