[A]: Light is very important external factor of transpiration.
[R]: Light induces stomata! opening and darkness stomata! closure. Thus the rate of transpiration increases in light and de- creases in dark.

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 "light is a very important external factor of transpiration." This means that light plays a significant role in the process of transpiration in plants.

2. **Understanding the Reason**: The reason provided is that "light induces stomatal opening and darkness causes stomatal closure." This indicates that the presence of light affects the behavior of stomata, which are small openings on the leaves of plants.

3. **Analyzing the Effects of Light and Darkness**:
- **In Light**: When light is present, stomata open, allowing water vapor to escape from the plant, which increases the rate of transpiration.
- **In Darkness**: Conversely, in the absence of light, stomata tend to close, reducing the rate of transpiration.

4. **Connecting Assertion and Reason**: Since the assertion claims that light is important for transpiration and the reason explains how light affects stomatal behavior (opening in light and closing in darkness), we can see that the reason supports the assertion.

5. **Conclusion**: Both the assertion and the reason are true, and the reason correctly explains why light is an important factor in transpiration. Therefore, the correct answer is that both the assertion and reason are true, and the reason is the correct explanation of the assertion.

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

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