最新完整托福模拟题阅读

最新完整托福模拟题阅读

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时间:2019-06-27

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

2、heliumexistInsuchsmallquantitiesthatitisaccuratetosaythattheuniversesomewhatmorethan25percentheliumbyweightandsomewhatlessthan25percenthydrogen.  Astronomershavemeasuredtheabundanceofheliumthroughoutourgalaxyandinothergalaxiesaswell.HeliumhasbeenfoundInoldstars,

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

4、'tseemtomakeverymuchdifferencewheretheheliumisfound.Itsrelativeabundanceneverseemstovarymuch.Insomeplaces,theremaybeslightlymoreofit;Inothers,slightlyless,buttheratioofheliumtohydrogennucleialwaysremainsaboutthesame.  Heliumiscreatedinstars.Infact,nuclearreactio

5、nsthatconverthydrogentoheliumareresponsibleformostoftheenergythatstarsproduce.However,theamountofheliumthatcouldhavebeenproducedinthismannercanbecalculated,anditturnsouttobenomorethanafewpercent.Theuniversehasnotexistedlongenoughforthisfiguretohesignificantlygre

6、ater.Consequently,iftheuniverseissomewhatmorethan25percentheliumnow,thenitmusthavebeenabout25percentheliumatatimenearthebeginning..  However,whentheuniversewaslessthanoneminuteold,noheliumcouldhaveexisted.Calculationsindicatethatbeforethistimetemperaturesweretoo

7、highandparticlesofmatterweremovingaroundmuchtoorapidly.Itwasonlyaftertheone-minutepointthatheliumcouldexist.Bythistime,theuniv9  ersehadcooledsufficientlythatneutronsandprotonscouldsticktogether.Butthenuclearreactionsthatledtotheformationofheliumwentonforonlyare

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

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