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1、选修7U20一、阅读理解AOldFossilsScientistshavefoundwhattheythinkisprobablytheoldestfossilonEarth,aremnantoflifefrom3.7billionyearsagowhenEarth’sskieswereorangeanditsoceansgreen.InanewlymeltedpartofGreenland,Australianscientistsfoundtheleftoverstructurefromamunityofmicrobes（微生物）thatlivedonana
2、ncientseafloor.Basedontheiranalysisofthefossils,theresearchersdeterminedthattheyare220millionyearsolderthanthosediscoveredinWesternAustralia,whichwere3.48billionyearsold.Thediscoveryshowslifemayhaveformedquickerandmoreeasilythanoncethought,abouthalfabillionyearsafterEarthformed.Andt
3、hatmayalsogivehopeforlifeformingelsewhere,suchasMars,saidstudyco-authorMartinVanKranendonkoftheUniversityofNewSouthWalesanddirectoroftheAustralianCenterforAstrobiology.＂Itgivesusanideahowourplanetevolvedandhowlifegainedafoothold,＂VanKranendonksaid.Scientistshadthoughtitwouldtakeatle
4、asthalfabillionyearsforlifetoformafterthemoltenEarthstartedtocoolabit,butthisshowsitcouldhavehappenedquicker,hesaid.That’sbecausethenewlyfoundfossilisfartooplextohavedevelopedsoonaftertheplanet’sfirstlifeforms,hesaid.Inanoutcropofrocksthatusedtobecoveredwithiceandsnowwhichmeltedafte
5、ranexceptionallywarmspring,theAustralianteamfoundstromatolites（叠层石）,whicharelayeredstructuresthatareoftenproducedbyamunityofmicrobes.Thestromatoliteswereabout1to4centimetershigh.＂Itislikethehouseleftbehindmadebythemicrobes,＂VanKranendonksaid.＂Scientistsusedthelayersofashfromvolcanoe
6、sandtinyzircon（锆石）withuraniumandestimatedthattheydatebackto3.7billionyearsago.＂Thedatingseemsaboutright,saidAbigailAllwood,aNASAastrobiologistwhofoundthepreviousoldestfossil,from3.48billionyearsago,inAustralia.ButAllwoodsaidsheisnotpletelyconvincedthatwhatVanKranendonk’steamfoundonc
7、ewasalive.Shesaidtheevidencewasn’tconclusiveenoughthatitwaslifeandnotageologicquirk（地质巧合）.＂Itwouldbenicetohavemoreevidence,butintheserocksthat’salottoask,＂Allwoodsaidinanemail.1.Theunderlinedwords＂gainedafoothold＂inParagraph3mean______.A.continuedB.changedC.increasedD.Started2.Wecan
8、learnfromParagraph4
