Since the energy is available only during the day and only then when there is not a great deal of cloud cover, the heat must be stored. This can be done in wa ter, or in rock or pebble beds. The energy is stored in the hot water in the pipes.
21. Solar energy was first used_______.
A. to start a fire B. to cook food
C. to power a heating-system D. in a war
22. What must be done to solar energy before it can be used?
A. It must be stored in rock beds. B. It must be distilled.
C. It must be converted to heat. D. It must be purified.
23. There is so much research into the use of solar energy because _______.
A. more irrigation projects are needed B. the construction industry is expanding C. fossil fuels are running out D. it transmits electromagnetic radiation
24. Solar energy must be stored because_______.
A. it is limited B. there is not much of it
C. it is not always available D. the passage does not tell us why
25. How many uses has solar energy been put to according to the passage?
A. 2. B. 3.
C. 4. D. More than 4.
答案:21. D 22. C 23. C 24. C 25. B
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