新颖完整托福模拟题阅读

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1、实用文档最新完整托福模拟题阅读(一)Questions12-20TheelementsotherthanhydrogenandheliumexistInsuchsmallquantitiesthatitisaccuratetosaythattheuniversesomewhatmorethan25percentheliumbyweightandsomewhatlessthan25percenthydrogen.AstronomershQuestions12-20　　Theelementsother

2、thanhydrogenandheliumexistInsuchsmallquantitiesthatitisaccuratetosaythattheuniversesomewhatmorethan25percentheliumbyweightandsomewhatlessthan25percenthydrogen.　　Astronomershavemeasuredtheabundanceofheliumthroughoutourgalaxyandinothergalaxiesaswell.Hel

3、iumhasbeenfoundInoldstars,inrelativelyyoungones,ininterstellargas,andinthedistantobjectsknownasquasars.Heliumnucleihavealsobeenfoundtobeconstituentsofcosmicraysthatfallontheearth(cosmic"rays"arenotreallyaformofradiation;theyconsistofrapidlymovingparti

4、clesofnumerousdifferentkinds).Itdoesn'tseemtomakeverymuchdifferencewheretheheliumisfound.Itsrelativeabundanceneverseemstovarymuch.Insomeplaces,theremaybeslightlymoreofit;Inothers,slightlyless,buttheratioofheliumtohydrogennucleialwaysremainsaboutthesam

5、e.　　Heliumiscreatedinstars.Infact,nuclearreactionsthatconverthydrogentoheliumareresponsibleformostoftheenergythatstarsproduce.However,theamountofheliumthatcouldhavebeenproducedinthismannercanbecalculated,anditturnsouttobenomorethanafewpercent.Theunive

6、rsehasnotexistedlongenoughforthisfiguretohesignificantlygreater.Consequently,iftheuniverseissomewhatmorethan25percentheliumnow,thenitmusthavebeenabout25percentheliumatatimenearthebeginning..　　However,whentheuniversewaslessthanoneminuteold,noheliumcoul

7、dhaveexisted.Calculationsindicatethatbeforethistimetemperaturesweretoohighandparticlesofmatterweremovingaroundmuchtoorapidly.Itwasonlyaftertheone-minutepointthatheliumcouldexist.Bythistime,theuniv文案大全实用文档　　ersehadcooledsufficientlythatneutronsandproto

8、nscouldsticktogether.Butthenuclearreactionsthatledtotheformationofheliumwentonforonlyarelativelyshorttime.Bythetimetheuniversewasafewminutesold,heliumproductionhadeffectivelyceased.　　12.whatdoesthepassagemainlyexplain?　　(A)Howstarsproduceenerg