Water is used as a moderator in nuclear reactor.
Moderator is a light substance that absorb neutrons.

Options

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

Explanation

To solve the question regarding the use of water as a moderator in nuclear reactors, we can break down the information provided into clear steps:

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "Water is used as a moderator in nuclear reactors."
- This is true because water is commonly used in many types of nuclear reactors to slow down fast neutrons produced during fission reactions.

2. **Understanding the Reason**: The reason states that "A moderator is a light substance that absorbs neutrons."
- This statement is also true. Moderators are indeed typically light materials (like water, graphite, or beryllium) that slow down neutrons by colliding with them.

3. **Function of a Moderator**: The role of a moderator in a nuclear reactor is to slow down the neutrons produced from fission.
- Slowing down the neutrons increases the likelihood of these neutrons interacting with the nuclear fuel (like uranium-235 or plutonium-239), which is essential for sustaining the chain reaction.

4. **Conclusion**: Since both the assertion and the reason are true, and the reason correctly explains why water is used as a moderator, we conclude that:
- Both the assertion and the reason are true, and the reason is the correct explanation of the assertion.

### Final Answer:
A. Both A and R are true, and R is the correct explanation of A.

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