Solar equipment Since about 1800, near the beginning of the Industrial Revolution, coal, petroleum, and gas have been used at a

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问题 Solar equipment
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(51) is limited, and electric power is very scarce in many places. Some scientists believe solar energy is the only(52) which can meet the world’s enormous(53) for power. Great progress had been made in harnessing the sun. Not(54) is the sun now used in cooking, but it also supplies power for(55) things as beacon lights for ships and airplanes. It operates telephone lines, portable radios, electric clocks, hearing aids, and(56) communication satellites. Some homes and office buildings in the United States are now being(57) with solar energy.
The three most significant types of solar equipment so(58) are the furnace, the still, and the cell. Solar furnaces heat water or air,(59) is then circulated through a building to make it comfortable. Solar stills are(60) important because they can provide(61) water at a relatively cheap rate. Salt can be easily removed from sea water with a solar still.
The most highly developed is the solar cell. Some cells are so(62) that they can turn 16 percent of the energy they received from the sun into electric energy. One of the cell’s biggest advantages is(63) it can be made either small enough to carry(64) large enough to produce a current that can run an automobile. Besides, it has a very long life. It is still too expensive for the average consumer. But when ways are(65) to produce it more cheaply, we can expect its use to be widespread.

选项 A、that
B、what
C、which
D、it

答案C

解析 此处要求填一个关系代词来引导一个非限制性定语从句修饰 water or air。四个选项中只有which可引导限制性定语从句,由此可得出正确答案。
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