Fill in the blanks. 

(a) Fossil fuels are [[1]] , [[2]] and [[3]]. 

(b) Process of separation of different constituents from petroleum is called [[4]] . 

(c) Least polluting fuel for vehicle is [[5]] .

(a) coal, petroleum and natural gas.

 A fossil fuel is a fuel formed by natural processes, such as anaerobic decomposition of buried dead organisms, containing organic molecules originating in ancient photosynthesis that release energy in combustion. Fossil fuels are coal, petroleum and natural gas.

(b) Refining.

Process of separation of different constituents from petroleum is called Refining

(c)  CNG (compressed natural gas).

CNG emits significantly less pollution directly than gasoline or oil when combusted. Hence least polluting fuel for vehicles is CNG (compressed natural gas).