2. 外语
  3. Since about 1800, near the beginning of the industrial revolution, coal, petroleum and gas have been used at a rapidly increasin

Since about 1800, near the beginning of the industrial revolution, coal, petroleum and gas have been used at a rapidly increasin

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问题     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.
【C17】
选项 A、how
B、that
C、why
D、when
答案B
解析 此处需要连接词引导表语从句，从句中结构完整，不缺成分，故选B)。
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