A hard material X which is mined from the earth is used as a household fuel and also for the generation of electricity

A hard material X which is mined from the earth is used as a household fuel and also for the generation of electricity at Thermal Power Stations. A soft material Y is also used as a fuel in the form of candles. A gaseous material Z which occurs along with petroleum is increasingly being used as a fuel in running vehicles in its compressed form.

(a) What are materials, X, Y and Z ?

(b) When materials X, Y and Z are burned separately :

(i) Which material burns by producing a yellow, luminous flame ?

(ii) Which material ultimately burns without producing a flame ?

(iii) Which material can burn in a gas stove by producing a blue flame ?