(i) Petroleum or crude oil is called black gold and it is formed from the remains of plants and animals that lived in the sea millions of years ago. The dead remains along with the silt which settled in the sea bed are attacked by bacteria and got converted to crude oil and gas under high pressure conditions. Slowly the silt compressed into rocks and the oil seeped into the porous parts of the rocks.

(ii) Fossil fuels such as coal and petroleum contain some amount of nitrogen and sulphur in them. The combustion of these fuels in air results in the formation of oxides of nitrogen and sulphur which are major pollutants of the environment. Therefore, coal and petroleum are considered to be pollutants of air.