A.This is because the activation energy for the combustion reaction is very high. On applying flame to the fuel, the part of the fuel and air in contact with the flame absorb heat which provides the necessary activation energy and the combustion starts. The heat liberated provides activation energy to the remaining fuel and hence the combustion continues. Thus, fuels which are thermodynamically unstable are kinetically stable (i.e., from chemical kinetics point of view). In other words, the stability of the fuels is due to the existence of energy barriers between the reactants and the products. These energy barriers have helped in the existence of life on the earth because if these barriers were not present, all the fuels on this earth would have burnt away by themselves.