Different methods of extinguishing a fire involve the following :
I. Removing the combustible substance
II. Cooling the substance to a temperature below its ignition temperature
III. Cutting off the supply of air
Which of the following statements is correct ?

Options

A. Method I is suitable in case of a building on fire.
B. Method II involves water which works when wood or paper are on fire.
C. Method III involves carbon dioxide gas which is lighter than oxygen and forms a blanket around the air.
D. All of these

Explanation

To determine which statements about methods of extinguishing a fire are correct, let's analyze each method one by one.

### Step 1: Analyze the First Method
**Statement I: Removing the combustible substance**
- This method suggests that to extinguish a fire, one should remove the material that is burning (the combustible substance).
- In practice, this is often not feasible, especially in larger fires such as those in buildings, where the entire structure may be involved. Removing the combustible material (like walls, furniture, etc.) is impractical and difficult.
- **Conclusion**: This statement is incorrect.

### Step 2: Analyze the Second Method
**Statement II: Cooling the substance to a temperature below its ignition temperature**
- The ignition temperature is the minimum temperature at which a substance will ignite and start burning.
- If we cool the burning material below this temperature, it will not be able to sustain combustion.
- This method is effective, especially for materials like wood or paper. For example, using water to cool these materials can help extinguish the fire.
- **Conclusion**: This statement is correct.

### Step 3: Analyze the Third Method
**Statement III: Cutting off the supply of air**
- Fire requires oxygen to sustain combustion. By cutting off the air supply, we can extinguish the fire.
- One common way to do this is by using carbon dioxide (CO2) extinguishers, which displace oxygen and form a blanket around the fire, preventing it from getting the oxygen it needs.
- Since CO2 is not combustible and does not support combustion, this method is effective.
- **Conclusion**: This statement is correct.

### Final Conclusion
Based on the analysis:
- Statement I is incorrect.
- Statement II is correct.
- Statement III is correct.

Thus, the correct statements regarding methods of extinguishing a fire are **II and III**.