题目内容：
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 67 our protective blanket on 68 . Light gets through, and this is essen- tial 69 plants to make the food which we 70 Heat, 71 , makes our environments tolerable and some ultraviolet rays (紫外线) penetrate the 72 . Cosmic ( 宇宙的) rays of various kinds come 73 the air from outer space, but 74 quantities of radiation from the sun arescreened off. As 75 as men leave the atmosphere they are 76 to this radiation; 77 their spacesuits or the walls of their spacecraft, if they are inside, 78 prevent a lot of radiation damage.
Radiation is the greatest known danger to explorers in 79 . Doses of radiation are measured in 80 called "rems (雷姆 )". We all 81 radiation here on earth from the sun, from cosmic rays and from radioactive minerals.
The " 82 " dose of radiation that we receive each year is about 100 millirems; it 83 according to where you live, and this is a very rough estimate. Scientists have reason to think 84 a man can 85 up with far more radia- tion 86 this without being damaged; the figure of 60 rems has been agreed.
阅读以上文章，回答题请在（67）处填上最佳答案。 A．for
B．with
C．on
D．as
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答案解析：