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] 【C3】

选项 A、when
B、where
C、which
D、what

答案 C

解析考查主从句语义逻辑。通过分析句子结构可知，该空后接的成分为定语从句，修饰先行词food，且所缺的引导词在从句中作eat的宾语，which符合此处语境，故选C。when和where引导定语从句时，分别充当时间状语和地点状语，不符合语境，what不能引导定语从句。

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