Assertion : The contraction and relaxation of muscle fiber are controlled by nerve impulses.
Reason : The threshold stimulus is the minimum stimulus required for the beginning of contraction.

Options

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

Answer

To analyze the assertion and reason provided in the question, we can break down the solution into the following steps:

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "the contraction and relaxation of muscle fiber are controlled by nerve impulses."
- This is true because muscle fibers contract and relax in response to signals from motor neurons, which transmit nerve impulses.

2. **Understanding the Reason**:
- The reason states that "the threshold stimulus is the minimum stimulus required for the beginning of contraction."
- This is also true because the threshold stimulus is the minimum level of stimulation needed to trigger an action potential in the muscle fibers, leading to contraction.

3. **Linking Assertion and Reason**:
- While both statements are true, we need to determine if the reason directly explains the assertion.
- The assertion describes the role of nerve impulses in muscle contraction and relaxation, while the reason explains a specific aspect of how contraction begins (the threshold stimulus).

4. **Conclusion**:
- Since both the assertion and reason are true, but the reason does not directly explain the assertion, the correct option is that both are true but the reason is not the correct explanation of the assertion.

### Final Answer:
- The correct option is **B**: Both assertion and reason are true, but the reason is not the correct explanation of the assertion.

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