Since about 1800, near the beginning of the industrial revolution, coal, petroleum and gas

have been used at a rapidly increasing rate. The supply of these【C1】______is limited and electric power is very【C2】______in many places. Some scientists believe solar energy is the only【C3】______that can meet the world's enormous【C4】______for power. Great progress had been made harnessing the sun. Not only is the sun now used in cooking, 【C5】______it also supplies power for【C6】______things as beacon lights for ships and airplanes. It【C7】______telephone lines, portable radios, electric clocks, hearing aids, and even communication satellites. Some homes and office buildings in U. S. A. are now being【C8】______with solar energy. The three most【C9】______types of solar equipment so far are the furnace, the still(蒸馏器)and the cell. Solar furnaces heat water or air,【C10】______is then circulated through a building to make it comfortable. Solar stills are【C11】______important because they can provide【C12】______water at a relative cheap rate. Salt can easily be【C13】______from sea water with solar still. The most highly developed type of solar【C14】______equipment is the solar cell. Some cells are so【C15】______that they can turn 16% of the energy they receive from the sun【C16】______electric energy. One of the cell's biggest advantages is【C17】______it can be made either small enough to carry【C18】______large enough to produce current that can run an automobile. Besides, it has a very long life. It is still too expensive for the【C19】______consumer but when ways are【C20】______to produce it more cheaply, we can expect its use to be widely-spread.

【C1】

A．materials

B．metals

C．reserves

D．fuels