Define the term 'fuel'. What are primary and secondary fuels? To which class of fuel do the following belong: Coke,Wood,Petroleum,$LPG$?

(N/A) fuel is any material that releases energy,typically in the form of heat,when it undergoes combustion or chemical reaction.
Primary Fuels: These are naturally occurring substances that are used directly as a source of energy without any significant processing. Examples include wood,coal,and petroleum.
Secondary Fuels: These are fuels derived from primary fuels through physical or chemical processing. They are often more convenient or efficient to use. Examples include coke,charcoal,and $LPG$.
Classification:
$(i)$ Wood and Petroleum are primary fuels.
$(ii)$ Coke and $LPG$ are secondary fuels.