The difference between combustible over non-combustible substance is:
(a).it cannot ignite
(b).it can ignite
(c).need low activation energy
(d).need high activation energy

Answer
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Hint: Some substances can burn in air and some cannot. Substances like wood, petrol, CNG and kerosene can burn easily, while substances like iron nails, stone, glass etc cannot combust when exposed to flame.

Complete answer:
Let us differentiate combustible and non-combustible substances.
Combustible substancesNon-combustible substances
When a substance can burn in air and produce heat and light is known as a combustible substance. Certain substances cannot be burnt in the presence of air. Such substances are called non-combustible substances.
Combustible substances can only burn if there are necessary ingredients such as heat, fuel and air.Even if a non-combustible substance is exposed to flame, it will not combust.
Some examples of combustible substances are diesel, petrol and kerosene. Examples of non-combustible substances are glass, stone, Portland cement concrete etc.

Combustion is a high temperature exothermic redox chemical reaction which takes place between a fuel and an oxidant. This oxidant is usually atmospheric oxygen. The reaction produces oxidised, gaseous products in a mixture and is termed as smoke. Combustion is not fully caused by fire. If it is caused by fire, then the flame can be the characteristic indicator for the reaction.
Therefore, the difference between combustible over non combustible is option (b) it can ignite.

Note: We often confuse combustible substances with flammable substances. Flammable substances are substances which are combustible and ignite very easily at ambient temperatures. That is, combustible substances ignite with some effort while flammable substances can catch fire easily on exposure to flame.