题目内容：
根据以下材料，回答题Spaceisadangerousplace,notonlybecauseofmeteorsbutalsobecauseofraysfromthesunandotherstars.Theatmosphereagainactsasourprotectiveblanketonearth.Lightgetsthrough,andthisisessentialforplantstomakethefoodwhichweeat.Heat,too,makesourenvironmentendurable.Variouskindsofrayscomethroughtheairfromouterspace,butenormousquantitiesofradiationfromthesunarescreenedoff.Assoonasmenleavetheatmospheretheyareexposedtothisradiation.Buttheirspacesuitsorthewallsoftheirspacecraft,iftheyareinside,dopreventalotofradiationdamage.
Radiationisthegreatestknowndangertoexplorersinspace.Theunitofradiationiscalled
"rem".Scientistshavereasontothinkthatamancanputupwithfarmoreradiationthan0.1rem
withoutbeingdamaged;thefigureof60remshasbeenagreedon.Thetroubleisthatitisextremelydifficulttobesureaboutradiationdamage-apersonmayfeelperfectlywell,butthecellsofhisorhersexorgansmaybedamaged,andthiswillnotbediscovereduntilthebirthofdeformed(畸形的)childrenorevengrandchildren.MissionsoftheApolloflightshavehadtocrossbeltsofhighradiationand,duringtheoutwardandreturnjoumeys,theApollocrewaccumulatedalargeamountofrems.Sofar,nodangerousamountsofradiationhavebeenreported,buttheApollomissionshavebeenquiteshort.Wesimplydonotknowyethowmenaregoingtogetonwhentheyspendweeksandmonthsoutsidetheprotectionoftheatmosphere,workinginaspacelaboratory.Drugsmighthelptodecreasethedamagedonebyradiation,butnoreallyeffectiveoneshavebeenfoundsofar.
Accordingtothefirstparagraph,theatmosphereisessentialtomaninthat_. A．itprotectshimagainsttheharmfulraysfromspace
B．itprovidessufficientlightforplantgrowth
C．itsuppliestheheatnecessaryforhumansurvival
D．itscreensoffthefallingmeteors
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