Assertion : Alveoli are the primary sites for exchange of gases.
Reason : All factors in our body are favourable for diffusion of `O_(2)` from alveoli to tissues and that of `CO_(2)` from tissues to alveoli.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion
B. If both assertion and reason are true and reason is not the correct explanation of assertion
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Explanation

To solve the question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "Alveoli are the primary sites for exchange of gases."
- This is true because alveoli are tiny air sacs in the lungs where the exchange of oxygen and carbon dioxide occurs. They provide a large surface area and are surrounded by capillaries, facilitating gas exchange.

2. **Understanding the Reason**:
- The reason states that "All factors in our body are favorable for diffusion of O2 from alveoli to tissues and that of CO2 from tissues to alveoli."
- This is also true. The movement of gases is driven by differences in partial pressure. Oxygen diffuses from areas of higher partial pressure (in the alveoli) to areas of lower partial pressure (in the tissues), while carbon dioxide diffuses in the opposite direction.

3. **Evaluating the Relationship**:
- The reason explains why the assertion is true. The favorable conditions for gas diffusion (due to partial pressures) support the assertion that alveoli are the primary sites for gas exchange.

4. **Conclusion**:
- Both the assertion and the reason are true, and the reason provides a correct explanation for the assertion. Therefore, the correct answer is that both the assertion and reason are true, and the reason is the correct explanation of the assertion.

### Final Answer:
Both the assertion and reason are true, and the reason is the correct explanation of the assertion.

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