2013英语四级完型填空练习题第029组

英语自学网

　　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 --1-- our protective blanket on --2--. Light gets through, and this is essential --3-- plants to make the food which we --4--. Heat, --5--, makes our environments tolerable and some ultraviolet rays(紫外线的) penetrate the --6--. Cosmic(宇宙的) rays of various kinds come --7-- the air from outer space, but --8-- quantities of radiation from the sun are screened off. --9-- men leave the atmosphere they are --10-- to this radiation; --11-- their spacesuits or the walls of their spacecraft, if they are inside, --12-- prevent a lot of radiation damage. Radiation is the greatest known danger to explorers in --13--. Doses of radiation are measured in --14-- called "rems(雷目)". We all --15-- radiation here on earth from the sun, from cosmic rays and from radioactive minerals. The "--16--" dose of radiation that we receive each year is about two millirems; it --17-- according toswheresyou live, and this is a very rough estimate. Scientists have reason to think --18-- a man can --19-- far more radiation --20-- without being damaged; the figure of 60 rems has been agreed.