Assertion : A body that is a good radiator is also a good absorber of radiation at a given wavelength.
Reason : According to Kirchhoff's law the absorptivity of a body is equal to its emissivity at a given wavelength

Options

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

Explanation

To solve the question, we need to analyze the assertion and the reason provided, and determine their validity based on the principles of physics, particularly Kirchhoff's law.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "A body that is a good radiator is also a good absorber of radiation at a given wavelength."
- This means that if a material can effectively emit radiation (as a good radiator), it should also be able to absorb radiation efficiently at the same wavelength.

2. **Understanding the Reason**:
- The reason provided is "According to Kirchhoff's law, the absorptivity of a body is equal to its emissivity at a given wavelength."
- Kirchhoff's law states that for a body in thermal equilibrium, the amount of radiation it emits (emissivity) is equal to the amount of radiation it absorbs (absorptivity) at a specific wavelength.

3. **Applying Kirchhoff's Law**:
- From Kirchhoff's law, we can conclude that if a body has high emissivity (is a good radiator), it must also have high absorptivity (is a good absorber) at that wavelength.
- Therefore, both the assertion and the reason are consistent with each other.

4. **Conclusion**:
- Since both the assertion and the reason are true, and the reason correctly explains the assertion, we can conclude that the statement is valid.
- The correct answer is that both the assertion and the reason are true, and the reason is the correct explanation of the assertion.

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

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