Assertion:In a pressure cooker the water is brought to boil.The cooker is then removed from the stove.Now on removing the lid the pressure cooker,the water starts boiling again.
Reason:The impurities in water bring down its boiling point.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are truw but reason is not correct explanation of assertion
C. If assertion is true but reason is false
D. If both assertion and reason are false.

Explanation

To solve the problem, we need to analyze the assertion and the reason provided in the question.

### Step-by-step Solution:

1. **Understanding the Assertion**:
- The assertion states that when water in a pressure cooker is brought to a boil and then the cooker is removed from the stove, upon removing the lid, the water starts boiling again.
- This is true because when the pressure cooker is heated, the pressure inside increases, allowing water to boil at temperatures higher than 100°C. When the cooker is removed from the heat, the pressure begins to drop.

2. **Understanding the Reason**:
- The reason given is that impurities in water bring down its boiling point.
- This statement is partially true, as impurities can affect the boiling point of water. However, the main reason for the water boiling again after removing the lid is due to the decrease in pressure, not because of impurities.

3. **Analyzing the Relationship**:
- The assertion is true because the water does boil again when the lid is removed due to the drop in pressure.
- The reason is false in the context of the assertion because it does not correctly explain why the water boils again after the lid is removed.

4. **Conclusion**:
- Since the assertion is true and the reason is false, the correct answer is that the assertion is true but the reason is false.

### Final Answer:
- The correct option is: **Third: If assertion is true but reason is false.**