Fill in the blanks.
A. Fossil fuels are ______, ______ and __________ .
B. Process of separation of different constituents from petroleum is called_________ .
C. Least polluting fuel for vehicles is __________.

Hint- In order to fill the blanks we will look for different options and find the suitable one. For the first case we will look for different types of fossil fuel and would name 3 of them. For the second one we look for the process involved and for the third case we will compare different fuels to find the least polluting fuel.

Complete answer:
The answers to the blanks are:
A. Fossil fuels are coal, petroleum and natural gas.
B. Process of separation of different constituents from petroleum is called Refining.
C. Least polluting fuel for vehicles is CNG (compressed natural gas).

Note- 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. They are exhaustible and also a major source of pollution. CNG emits significantly less pollution directly than gasoline or oil when combusted. Although research is still going on to replace CNG with hydrogen for less pollution.