[A): DNA replicates after mitosis.
[R]: In mitosis, the chromosomal number does not stay constant

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

To analyze the given assertion and reason, we need to break down the statements and understand the processes involved in the cell cycle, particularly focusing on mitosis and DNA replication.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states: "DNA replicates after mitosis."
- In the context of the cell cycle, DNA replication occurs during the S phase, which is part of interphase, before mitosis begins. Therefore, this assertion is **false**.

2. **Understanding the Reason (R)**:
- The reason states: "In mitosis, the chromosomal number does not stay constant."
- Mitosis is a process where a single cell divides to produce two identical daughter cells, maintaining the same chromosomal number as the parent cell. Therefore, this statement is also **false**.

3. **Conclusion**:
- Since both the assertion and the reason are false, the correct answer to the question is that both A and R are false.

### Final Answer:
Both the assertion (A) and the reason (R) are false.

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