The bulb of a thermometer when dipped in petrol and then taken out, the level of the mercury thread in the thermometer.

Options

A. starts falling
B. starts rising
C. remians at the same level
D. initially falls and then rises

Explanation

To solve the question regarding what happens to the level of mercury in a thermometer when its bulb is dipped in petrol and then taken out, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Situation**:
- When the bulb of a thermometer is dipped in petrol, the surface of the bulb comes into contact with the petrol.

2. **Evaporation of Petrol**:
- After removing the thermometer from the petrol, the petrol that remains on the surface of the bulb will start to evaporate due to the surrounding heat.

3. **Cooling Effect of Evaporation**:
- The process of evaporation requires energy, which is taken from the surface of the thermometer bulb. This results in a cooling effect on the bulb.

4. **Effect on Mercury Level**:
- As the bulb cools down due to the evaporation of petrol, the mercury inside the thermometer will also cool down. Since the mercury contracts when cooled, the level of mercury in the thermometer will fall.

5. **Conclusion**:
- Therefore, when the bulb of the thermometer is dipped in petrol and then taken out, the level of the mercury thread in the thermometer will fall.

### Final Answer:
The level of the mercury thread in the thermometer will fall.

---