Assertion:Heavy water preffered over ordinary water as a moderator in reactors.
Reason:Heavy water,used for slowing down the neurons,has lesser absorption probability of neutrons than ordinary water.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are truw but reason is not 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 the assertion and reason given in the problem.

### Step 1: Understand the Assertion
The assertion states that "Heavy water is preferred over ordinary water as a moderator in reactors."
- **Explanation**: In nuclear reactors, moderators are materials used to slow down fast-moving neutrons produced during fission. Heavy water (D2O) is indeed preferred over ordinary water (H2O) in certain types of reactors (like CANDU reactors) because it is more effective in slowing down neutrons without absorbing them.

### Step 2: Understand the Reason
The reason states that "Heavy water, used for slowing down the neutrons, has lesser absorption probability of neutrons than ordinary water."
- **Explanation**: Heavy water has a lower neutron absorption cross-section compared to ordinary water. This means that heavy water is less likely to absorb neutrons, allowing more neutrons to be available for sustaining the nuclear chain reaction.

### Step 3: Evaluate the Statements
Both the assertion and the reason are true:
- Heavy water is indeed preferred over ordinary water as a moderator because it effectively slows down neutrons while having a lower absorption probability.
- The reason provided correctly explains why heavy water is advantageous in this context.

### Conclusion
Since both the assertion and the reason are true, and the reason correctly explains the assertion, we conclude that the answer is:
- **Both the assertion and reason are true, and the reason is a correct explanation of the assertion.**

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

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