Sources of natural gas include all of the following except:

flyash

oil wells

biomass

landfills

fly ash

Oil well, biomass and landfill are the sources of the natural gas. But fly ash is not the source of natural gas because it is formed by the combustion of the coal during the electricity generation process.

Oil well is a boring inside the earth crust which is digg open to obtain crude oil such as petroleum but the natural gas which sometimes lies above the liquid petroleum also collected.

Biomass is the reservoir of organic matter it is main composition of all living organisms. The decaying matter is a form of biomass. This biomass releases natural gason decomposition.

Landfill is the area where the biodegradable waste is disposed off. The action of biological agents such as bacteria feed upon these wastes and release natural gas.

i think the answer is b