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　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. 
 
　　1.A.for B.with C.on D.as
　　2.A.stars B.sun C.earth D.space
　　3.A.with B.from C.under D.for
　　4.A.use B.live C.eat D.get
　　5.A.again B.also C.besides D.too
　　6.A.environment B.space C.atmosphere D.earth
　　7.A.across B.to C.from D.through
　　8.A.valid B.enormous C.various D.proper
　　9.A.As soon as B.As well as
　　C.As much as D.As possible as
　　10.A.shown B.exposed C.faced D.covered
　　11.A.but B.because C.so D.so that
　　12.A.get B.make C.have D.do
　　13.A.earth B.atmosphere C.space D.environment
　　14.A.pieces B.units C.parts D.elements
　　15.A.receive B.accept C.bring D.catch
　　16.A.conventional B.commonC.general D.normal
　　17.A.shifts B.converts C.modifies D.varies
　　18.A.what B.which C.that D.why
　　19.A.put up with B.keep up with
　　C.come up with D.catch up with
　　20.A.from B.than C.as D.away