2021年1月大学英语六级阅读理解真题及答案-1.docx

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2021年1月大学英语六级阅读理解真题及答案

Questions21to25arebasedonthefollowingpassage.Therearedesertplantswhichsurvivethedryseasonintheformofinactiveseeds.Therearealsodesertinsectswhichsurviveasinactivelarvae(幼虫).Inaddition,difficultasitistobelieve,therearedesertfishwhichcansurvivethroughyearsofdrought(干旱)intheformofinactiveeggs.Thesearetheshrimps(小虾)thatliveintheMojaveDesert,anintenselydryregioninthesouth-westoftheUnitedStateswhereshadetemperaturesofover50Careoftenrecorded.TheeggsoftheMojaveshrimpsarethesizeandhavetheappearanceofgrainsofsand.Whensufficientspringrainfallstoformalake,onceeverytwotofiveyears,theseeggshatch(孵化).Thenthewaterissoonfilledwithmillionsoftinyshrimpsaboutamillimetrelongwhichfeedontinyplantandanimalorganismswhichalsogrowinthetemporarydesertlake.Withinaweek,theshrimpsgrowfromtheiroriginal1millimetretoalengthofabout1.5centimetres.Throughoutthetimethattheshrimpsarerapidlymaturing,thewaterinthelakeequallyrapidlyevaporates.Therefore,fortheshrimpsitisaraceagainsttime.Bythetwelfthday,however,whentheyareabout3centimetrelong,hundredsoftinyeggsformontheunderbodiesofthefemales.Usuallybythistime,allthatremainsofthelakeisalarge,muddypatchofwetsoil.Onthethirteenthdayandthenext,duringthefinalhoursoftheirbrieflives,theshrimpslaytheireggsinthemud.Then,havingensuredthattheirspecieswillsurvive,theshrimpsdieasthelastofthewaterevaporates.Ifsufficientrainfallsthenextyeartoformanotherlake,theeggshatch,andonceagaintheshrimpspassrapidlythroughtheircycleofgrowth,adulthood,egg-laying,anddeath.Someyearsthereisinsufficientraintoformalake:inthiscase,theeggswillremaindormantforanotheryears,orevenlongerifnecessary.Very,veryoccasionally,perhapstwiceinahundredyears,sufficientrainfallstoformadeeplakethatlastsamonthormore.Inthiscase,thespeciespassesthroughtwocyclesofgrowth,egg-laying,anddeath.Thus,onsuchoccasions,thespeciesmultipliesconsiderably,whichfurtherensuresitssurvival.21.WhichofthefollowingistheMOSTdistinctivefeatureofMojaveshrimps?