All the useful energy at the surface of the earth comes from the activity of the sun. The sun heats and feeds mankind. Each year it【21】men with two hundred million tons of grain and【22】ten million tons of wood. Coal, oil natural gas, and all other【23】are stored-up energy from the sun. It was collected by the plants as carbon compounds and stored【24】ago. Even waterpower comes from the sun. Water【25】into vapor by the sun falls as rain. It runs【26】the mountains and is converted to electric power. Light transmits only the energy【27】comes from the sun' s outer layers, and【28】of this energy that is directed toward the earth never arrives. About【29】of it is absorbed by the atmosphere of the earth. In【30】,the earth itself gets only one half-billionth of the Sun' s entire output of rediant energy. (41)