单项选择题
| 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. |