[A]: Xanthium will not flower if the long dark period is interrupted by a single brief flash of light.
[R]: Xanthium is a long day plant where con- tinuous long loght period as essential for flowering.

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 solve the question regarding the assertion and reason about Xanthium, we will analyze both statements step by step.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "Xanthium will not flower if the long dark period is interrupted by a single brief flash of light."
- This implies that Xanthium requires an uninterrupted dark period for flowering to occur.

2. **Understanding the Reason**:
- The reason states that "Xanthium is a long day plant where continuous long light is essential for flowering."
- This suggests that Xanthium needs long periods of light to flower, which contradicts the assertion.

3. **Identifying the Type of Plant**:
- Xanthium is actually classified as a short day plant, meaning it requires a shorter duration of light (less than 12-15 hours) to initiate flowering.
- For short day plants, a long dark period is crucial, and any interruption (like a brief flash of light) can prevent flowering.

4. **Evaluating the Assertion**:
- The assertion is true because if the dark period is interrupted, it can indeed prevent flowering in Xanthium.

5. **Evaluating the Reason**:
- The reason is false because it incorrectly classifies Xanthium as a long day plant when it is actually a short day plant.

6. **Conclusion**:
- Therefore, the assertion is true, and the reason is false. The reason does not correctly explain the assertion.

### Final Answer:
- Assertion: True
- Reason: False