Assertion: Two chromatids remain held together at one point called centromere.
Reason: Chromosome is double at all points along its length excluding centromere.

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

Answer

To solve the question, we need to analyze the assertion and the reason provided, and determine their validity and relationship.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "Two chromatids remain held together at one point called centromere."
- This is true because during cell division, chromosomes consist of two identical sister chromatids that are joined together at a region known as the centromere.

2. **Understanding the Reason**:
- The reason states that "Chromosome is double at all points along its length excluding centromere."
- This is also true. The chromosome is made up of two chromatids (thus "double") along its length, but at the centromere, they are held together and appear as a single point of constriction.

3. **Evaluating the Relationship**:
- The assertion is true, and the reason is also true. Furthermore, the reason provides a correct explanation for the assertion, as it describes the structural nature of the chromosome and how the chromatids are arranged.

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

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

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