Assertion: Senescence is the time when age associated defects are manifested
Reason: Certain genes may be undergoing sequential switching on and off during one's life

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 and R is false
D. If both A and R are false

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "Senescence is the time when age-associated defects are manifested."
- **Explanation**: Senescence refers to the process of biological aging in multicellular organisms, which includes the gradual deterioration of bodily functions and the emergence of age-related defects.

2. **Understanding the Reason**: The reason states that "Certain genes may be undergoing sequential switching on and off during one's life."
- **Explanation**: This suggests that throughout an individual's life, certain genes are activated and deactivated at different times, which can influence various biological processes, including aging.

3. **Connecting Assertion and Reason**:
- The assertion is true because senescence indeed marks the period when age-related defects become apparent.
- The reason is also true because the regulation of gene expression (switching on and off) is a critical mechanism that can affect aging and the manifestation of defects associated with senescence.

4. **Evaluating the Relationship**:
- The reason provides a valid explanation for the assertion. The switching of genes can lead to the deterioration of bodily functions, which is characteristic of senescence.

5. **Conclusion**: Both the assertion and reason are true, and the reason explains the assertion. Therefore, 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 the reason are true, and the reason is the correct explanation of the assertion.

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