Assertion: When a ball collised elastically with a floor, it rebounds with the same velocity as with it strikes.
Reason: Momentum of earth + ball system remains constant.

Options

A. If both assertion and Reason are true and the Reason is the correct explanation of the assertion.
B. If both assertion and Reason are true but the reason is not the correct explanation of the 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 when a ball collides elastically with the floor, it rebounds with the same velocity as it strikes.
- In an elastic collision, both momentum and kinetic energy are conserved. Therefore, if a ball strikes the floor with a certain velocity (let's say \( v \)), it will rebound with the same velocity \( v \) if we assume no energy is lost to other forms (like heat or sound).

2. **Understanding the Reason**:
- The reason states that the momentum of the earth + ball system remains constant.
- In any collision, the total momentum of the system (in this case, the earth and the ball) is conserved. However, the earth is so massive compared to the ball that its velocity change is negligible. Thus, for practical purposes, we can say that the ball's momentum change is significant while the earth's change is not.

3. **Analyzing the Relationship**:
- The assertion is true because in a perfectly elastic collision, the ball does rebound with the same velocity it had before the collision.
- The reason is also true because the momentum of the earth + ball system does remain constant during the collision.

4. **Determining the Connection**:
- While both statements are true, the reason does not provide a correct explanation for the assertion. The assertion specifically refers to the behavior of the ball in an elastic collision, while the reason discusses the conservation of momentum in a broader context that includes the earth.

5. **Conclusion**:
- Since both the assertion and reason are true but the reason does not correctly explain the assertion, the correct answer is that both are true, but the reason is not the correct explanation for the assertion.

### Final Answer:
Both assertion and reason are true, but the reason is not the correct explanation of the assertion.