Space is a dangerous place, not only because of meteors (流星) but also becaus

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问题 Space is a dangerous place, not only because of meteors (流星) but also because of rays from the sun and other stars. The atmosphere again acts 【C1】_____ our protective blanket on earth. Light gets through, and this is 【C2】_____ for plants to make the food 【C3】_____ we eat. Heat, too, makes our environment endurable. Various kinds of rays 【C4】_____ the air from outer space, 【C5】_____ enormous quantities of radiation from the sun are 【C6】_____ off. As soon as men leave the atmosphere they are 【C7】_____to this radiation but their spacesuits or the walls of their spacecraft, 【C8】_____ they are inside, do prevent a lot of radiation damage.
Radiation is the greatest known 【C9】_____ to explorers in space. The unit of radiation is called "rem (雷姆)". Scientists have reason to think that a man can 【C10】_____ far more radiation 【C11】_____ 0.1 rem without being damaged; the figure of 60 rems has been agreed on. The trouble is that it is extremely 【C12】_____ to be sure about radiation damage—a person may feel perfectly well, but the 【C13】_____ of his or her sex organs may be damaged, and this will not be discovered 【C14】_____ the birth of deformed children or even grandchildren.
Missions of the Apollo flights have had to cross belts of high amount of rems. So far, no dangerous amounts of radiation have been 【C15】_____, but the Apollo missions have been quite 【C16】_____. We simply do not know yet how men are going to get on when they 【C17】_____ weeks and months outside the 【C18】_____ of the atmosphere, working in a space laboratory. Drugs might help to 【C19】_____ the damage done by radiation, but no really 【C20】_____ ones have been found so far. [br] 【C1】

选项 A、as
B、in
C、on
D、to

答案 A

解析考查介词的作用。acts as意为“充当，起……的作用”，符合句意，故选A。C较具干扰性，act on意为“对……起作用”，后接被作用的对象，用在这里上下文的意思则变为“大气层对保护毯起作用”，句意不通；act to“对待”不符合句意，可排除。

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