A : If the earth did not have an atmosphere, its average surface temperature would have been lower.
R : In the absence of atmosphere, the green house effect will be absent.

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion, then mark (1)
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion, then mark (2)
C. If assertion is true statement but Reason is false, then mark (3)
D. If both Assertion and Reason are false statements, then mark (4)

Answer

To solve the assertion-reason question, we will analyze both the assertion (A) and the reason (R) step by step.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states: "If the earth did not have an atmosphere, its average surface temperature would have been lower."
- This is based on the fact that the atmosphere plays a crucial role in regulating the Earth's temperature. Without an atmosphere, there would be no medium to trap heat, leading to a significant drop in temperature.

2. **Understanding the Reason (R)**:
- The reason states: "In the absence of atmosphere, the greenhouse effect will be absent."
- The greenhouse effect is a natural process where certain gases in the atmosphere (like CO2, methane, etc.) trap heat from the sun, preventing it from escaping back into space. Without an atmosphere, this effect would not occur, leading to lower temperatures.

3. **Connecting Assertion and Reason**:
- The assertion and reason are connected. The absence of an atmosphere directly leads to the absence of the greenhouse effect, which is responsible for maintaining the Earth's temperature.
- Therefore, if the greenhouse effect is absent, the average surface temperature of the Earth would indeed be lower.

4. **Conclusion**:
- Both the assertion (A) and the reason (R) are correct.
- The reason provided (R) correctly explains the assertion (A).

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