Answer: natural gas
Step-by-step explanation:
Natural gas is a hydrocarbon gas which occur under the surface of earth. The natural gas is primarily composed of methane. Also contain carbon dioxide, hydrogen, helium and sulfide. It is formed by the decomposition of organic matter generated after the decaying plants and animals materials. The organic matter remains exposed to heat and pressure under the surface of earth. It takes millions of years to form natural gas under the surface of earth.