最新完整托福模拟题阅读.doc

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1、最新完整托福模拟题阅读(一)Questions12-20TheelementsotherthanhydrogenandheliumexistInsuchsmallquantitiesthatitisaccuratetosaythattheuniversesomewhatmorethan25percentheliumbyweightandsomewhatlessthan25percenthydrogen.AstronomershQuestions12-20　　Theelementsotherthanhydrogenandhelium

2、existInsuchsmallquantitiesthatitisaccuratetosaythattheuniversesomewhatmorethan25percentheliumbyweightandsomewhatlessthan25percenthydrogen.　　Astronomershavemeasuredtheabundanceofheliumthroughoutourgalaxyandinothergalaxiesaswell.HeliumhasbeenfoundInoldstars,inrelatively

3、youngones,ininterstellargas,andinthedistantobjectsknownasquasars.Heliumnucleihavealsobeenfoundtobeconstituentsofcosmicraysthatfallontheearth(cosmic"rays"arenotreallyaformofradiation;theyconsistofrapidlymovingparticlesofnumerousdifferentkinds).Itdoesn'tseemtomakeverymu

4、chdifferencewheretheheliumisfound.Itsrelativeabundanceneverseemstovarymuch.Insomeplaces,theremaybeslightlymoreofit;Inothers,slightlyless,buttheratioofheliumtohydrogennucleialwaysremainsaboutthesame.　　Heliumiscreatedinstars.Infact,nuclearreactionsthatconverthydrogentoh

5、eliumareresponsibleformostoftheenergythatstarsproduce.However,theamountofheliumthatcouldhavebeenproducedinthismannercanbecalculated,anditturnsouttobenomorethanafewpercent.Theuniversehasnotexistedlongenoughforthisfiguretohesignificantlygreater.Consequently,iftheunivers

6、eissomewhatmorethan25percentheliumnow,thenitmusthavebeenabout25percentheliumatatimenearthebeginning..　　However,whentheuniversewaslessthanoneminuteold,noheliumcouldhaveexisted.Calculationsindicatethatbeforethistimetemperaturesweretoohighandparticlesofmatterweremovingar

7、oundmuchtoorapidly.Itwasonlyaftertheone-minutepointthatheliumcouldexist.Bythistime,theuniv9　　ersehadcooledsufficientlythatneutronsandprotonscouldsticktogether.Butthenuclearreactionsthatledtotheformationofheliumwentonforonlyarelativelyshorttime.Bythetimetheuniversewasa

8、fewminutesold,heliumproductionhadeffectivelyceased.　　12.whatdoesthepassagemainlyexplain?　　(A)Howstarsproduceenergy　　(B)Thedi