Space is a erous place, not only because of meteors(流星) 【B1】 because of rays from the sun and 【B2】 stars. The atmosphere again acts 【B3】 our protective blanket on 【B4】 Light gets through, and this is essential 【B5】 plants to make the food which we 【B6】 Heat, 【B7】, makes our environments tolerable and some ultraviolet (紫外线的) rays penetrate the 【B8】 Cosmic (宇宙的) rays of various kinds come 【B9】 the air from outer space, but 【B10】 quantities of radiation from the sun are screened off. 【B11】 men leave the atmosphere they are 【B12】 to this radiation but their spacesuits or the walls of their spacecraft, if they are inside, 【B13】 pr a lot of radiation damage. Radiation is the greatest known er to explorers in 【B14】 Doses of radiation are measured in 【B15】 called "rems(雷姆) ". We all 【B16】 radiation here on earth from the sun, from cosmic rays and from radioactive minerals. The 【B17】 dose of radiation that we receive each year is about two millirems; it 【B18】 according to where you live, and this is a very rough estimate. Scientists have reason to think 【B19】 a man can 【B20】 far more radiation than this without being damaged; the figure of 60 rems has been agreed.

A．what
B．which
C．that
D．why