【单选题】

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 (62) our protective blanket on (63) . Light gets through, and this is essential (64) plants to make the food which we (65) . Heat, (66) , makes our environments tolerable and some ultraviolet rays (紫外线) penetrate the (67) .Cosmic (宇宙的) rays of various kinds come (68) the air from outer space, but (69) quantities of radiation from the sun are screened off.As (70) as men leave the atmosphere they are (71) to this radiation; (72) their spacesuits or the walls of their spacecraft, if they are inside, (73) prevent a lot of radiation damage.

Radiation is the greatest known danger to explorers in (74) .Doses of radiation are measured in (75) called "rems (雷姆)". We all 76 radiation here on earth from the sun, from cosmic rays and from radioactive minerals. The " (77) "dose of radiation that we receive each year is about 100 millirems; it (78) according to where you live, and this is a very rough estimate. Scientists have reason to think (79) a man can (80) up with far more radiation (81) this without being damaged; the figure of 60 rems has been agreeD、
[A] stars [C] earth
[B] sun[D] space