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  3. Space is a dangerous place, not only because of meteors but also because of rays from the sun and other stars. The atmosphere ag

Space is a dangerous place, not only because of meteors but also because of rays from the sun and other stars. The atmosphere ag

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问题      Space is a dangerous place, not only because of meteors but also because of rays from the sun and other stars. The atmosphere again acts as our protective blanket on earth. Light gets through, and this is essential for plants to make the food which we eat. Heat, too, makes our environment endurable. Various kinds of rays come through the air from outer space, but enormous quantities of radiation from the sun are screened off. As soon as men leave the atmosphere they are exposed to this radiation but their space suits or the walls of their spacecraft, if they are inside, do prevent a’ lot of radiation damage.
     Radiation is the greatest known danger to explorers in space. The unit of radiation is called "rem". Scientists have reason to think that a man can put up with far more radiation than 0. 1 rem without being damaged, the figure of 60 rems has been agreed on. The trouble is that it is extremely difficult to be sure about radiation damage--a person may feel perfectly well, but the cells of his or her sex organs may be damaged, and this will not be discovered until the birth of deformed children or even grandchildren Missions of the Apollo flights have had to cross belts of high radiation and, during the outward and return journeys, the Apollo crew accumulated a large amount of rems. So far, no dangerous amounts of radiation have been reported, but the Apollo missions have been quite short. We simply do not know yet how men are going to get on when they spend weeks and months.
According to the first paragraph, the atmosphere is essential to man in that ______.
选项 A、it protects him against the harmful rays from space
B、it provides sufficient light for plant growth
C、it supplies the heat necessary for human survival
D、it screens off the falling meteors
答案A
解析 要点定位题。本题问的是大气层对人类的重要性主要体现在哪一方面。由文章第一段第二句和第四句“The atmosphere again acts as our protective blanket on earth...Various kinds of rays come through the air from outer space，but enormous quantities of radiation from the sun are screened off”可知，A为正确答案。从文中第一段可知，为
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