What is the difference between combustible over non-combustible substances?
A. It cannot ignite
B. It can ignite
C. Need low activation energy
D. Need high activation energy

Hint: The word combustible is related with combustion which in turn applies to substances which produces heat in presence of air.

Complete step by step answer:
We know combustible substances are defined as those substances which can burn in air to give off energy in the form of heat and light. The necessary conditions for combustion to take place are fuel, heat and air. On the other hand, non-combustible substances do not undergo the process of combustion in the presence of air to give off heat and light. Basically, giving off energy in the form of heat and light is known as ignition. Therefore, it can be said that non-combustible substances do not undergo ignition but combustible substances do produce heat and light and hence undergo ignition.
Whereas, activation energy which is the minimum quantity of energy that has to be supplied for combustion to take place is not the basis of differentiating combustible substances and non-combustible substances. A combustible substance usually has a low activation energy and can rapidly burn in air when it comes in contact with the oxygen present in the air. The process of combustions continues till sufficient oxygen is present. Once all the oxygen gets used up in the process of combustion, then no further combustion can be seen. And hence the substance stops igniting. Therefore, the presence of oxygen is a necessary condition for combustion.

So, the correct answer is “Option B”.

Note:
You should be clear with the meaning of ignition and must not confuse it with combustion. Ignition is simply the act of initiating the process of combustion.