Hint: A fire needs three things in order to exist, namely a source of energy (may be a spark or heat), a fuel (thing that burns) and a source of oxygen. We simply need to remove one of these three necessary requirements for putting out a fire. Oil has the ability to float on water.
> Fire can be termed as the rapid and energetic oxidation of any fuel. In a typical fire, the source of oxygen is generally the air available to the fuel from the atmosphere. If a fuel can accept the oxygen in the atmosphere very easily, then it is said to be flammable, and vice versa.
> A water molecule contains an atom of oxygen and two atoms of hydrogen. It is a completely oxidized substance and thus cannot be oxidized further. Hence water does not burn. Water smothers the fire by creating a barrier between the source of oxygen and the fuel. Also, the conversion of liquid water into steam absorbs heat and thus imparts an additional cooling effect that helps put out the fire easily.
> But in case fires are caused by oil or petrol, water cannot be used to put out or control the fire. We know that oil and petrol have densities much lesser than that of water and thus they float on the surface of water. As a result, water cannot successfully create a barrier between the fuel which in this case is oil or petrol, and air.
Thus, the correct option is B.
Note: Water if used to put out the fire vaporizes into the stream of oil, thereby expanding it and causing it to spit out more oil. This results in the flame becoming much bigger rather than controlling it.