The most suitable substance for putting out petrol fire is

Step 1: Analyze the question Petrol is a flammable liquid fuel (a Class B fire). We need the best way to put out this kind of fire.

Step 2: The concept A fire needs three things: fuel, oxygen, and heat (the fire triangle). Removing any one of them puts the fire out. For a petrol fire:

• Water (A): Petrol is less dense than water and does not mix with it. The petrol floats on top of the water and keeps burning, and the fire spreads as the water flows. So water is unsuitable and dangerous.
• Carbon(IV) oxide (B): CO₂ is denser than air and is not combustible. It blankets the fire, cuts off the oxygen supply (smothering), and also cools slightly. It leaves no residue, which is why CO₂ extinguishers are standard for flammable liquid and electrical fires.
• Fire blanket (C): It works well on small fires, such as a pan of cooking oil or a person's burning clothes, but it is not suited to a petrol fire, which can be large and spreading.
• Sand (D): It can smother a small fire, but it is less effective on burning liquid that spreads, and it is not the most suitable choice.

Step 3: Solution Smothering the fire without spreading the burning liquid is what we need. CO₂ does this most effectively and cleanly.

Step 4: Answer The correct option is B. carbon(IV) oxide.

Tip: Remember "Water for wood, CO₂ for oil and wires." Never use water on petrol, oil, or electrical fires.