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 question, we need to analyze the assertion (A) and the reason (R) provided:

**Assertion (A):** If the earth did not have an atmosphere, its average surface temperature would have been lower.

**Reason (R):** In the absence of atmosphere, the greenhouse effect will be absent.

### Step-by-Step Solution:

1. **Understanding the Assertion (A):**
- The assertion states that without an atmosphere, the Earth's average surface temperature would be lower. This is true because the atmosphere plays a crucial role in regulating temperature through the greenhouse effect. Without it, there would be no mechanism to trap heat.

2. **Understanding the Reason (R):**
- The reason states that the greenhouse effect would be absent without an atmosphere. This is also true. The greenhouse effect is primarily caused by greenhouse gases in the atmosphere that trap infrared radiation emitted by the Earth, preventing it from escaping into space.

3. **Connecting Assertion and Reason:**
- The reason (R) explains why the assertion (A) is true. Without the greenhouse effect, the heat that the Earth receives from the Sun during the day would escape back into space at night, leading to a significant drop in temperature.

4. **Conclusion:**
- Both the assertion and the reason are correct, and the reason correctly explains the assertion. Therefore, the correct answer is that both A and R are true, and R is the correct explanation for A.

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

---