Space is a dangerous place, not only because of meteors(流星) 【C1】______be-cause of rays from the sun and【C2】______stars. The atmosphere again acts【C3】______our protective blanket on【C4】______. Light gets through, and this is essential【C5】______plants to make the food which we【C6】______. Heat,【C7】______, makes our environments tolerable and some ultraviolet(紫外线的)rays penetrate the【C8】______. Cosmic(宇宙的)rays of various kinds come【C9】______the air from outer space, but【C10】______quantities of radiation from the sun are screened off.【C11】______men leave the atmosphere they are【C12】______to this radiation but their spacesuits or the walls of their spacecraft, if they are inside,【C13】______prevent a lot of radiation damage. Radiation is the greatest known danger to explorers in【C14】______. Doses of radiation are measured in【C15】______called "rems(雷姆)". We all【C16】______radiation here on earth from the sun, from cosmic rays and from radioactive minerals. The "【C17】______" dose of radiation that we receive each year is about too milli-rems; it【C18】______according to where you live, and this is a very rough estimate. Scientists have reason to think【C19】______a man can【C20】______far more radiation than this without being damaged; the figure of 60 rems has been agreed on.

【C1】

A．but also

B．ever so

C．as well

D．or so