1)petroleum and natural gas are formed in the__________
2)_______ refers to the things that exists freely in nature or human use
3) during the ____ period the earth was covered with swampy forest's
4)-------- is the mixture of hydrogen, methane, carbon monoxide, and other gases
5)natural gas mainly consists of_____
6)_____are the chemicals derived from petroleum and natural gas
7) the process of refining of petroleum is carried out in refineries and is called _____
8) _____ is used to generate power and to manufacture other fuels like coke and coal tar​