托福TPO38阅读原文+题目Word文档格式.docx

托福TPO38阅读原文+题目Word文档格式.docx

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　　atmospherewithoneimportantexception:

oxygen.Earthhadvirtuallynooxygen

　　initsatmospheremorethan4billionyearsago,buttheatmosphereisnow

　　approximately21percentoxygen.

　　Tracesofoxygenwereprobablygeneratedintheearlyatmospherebythe

　　breakdownofwatermoleculesintooxygenandhydrogenbyultravioletlight（a

　　processcalledphotodissociation）.Althoughthisisanimportantprocess,it

　　cannotbegintoaccountforthepresenthighlevelsofoxygenintheatmosphere.

　　Almostallofthefreeoxygennowintheatmosphereoriginatedthrough

　　photosynthesis,theprocesswherebyplantsuselightenergytoinducecarbon

　　dioxidetoreactwithwater,producingcarbohydratesandoxygen.

　　Oxygenisaveryreactivechemical,soatfirstmostofthefreeoxygen

　　producedbyphotosynthesiswascombinedwithironinoceanwatertoformiron

　　oxide-bearingminerals.Theevidenceofthegradualtransitionfromoxygen-poor

　　tooxygen-richwaterispreservedinseafloorsediments.Themineralsin

　　seafloorsedimentaryrocksthataremorethanabout2.5billionyearsold

　　containreduced（oxygen-poor）ironcompounds.Inrocksthatarelessthan1.8

　　billionyearsold,oxidized（oxygen-rich）compoundspredominate.Thesediments

　　thatwereprecipitatedduringthetransitioncontainalternatingbandsofred

　　（oxidizediron）andblack（reducediron）minerals.Theserocksarecalled

　　banded-ironformations.Becauseoceanwaterisinconstantcontactwiththe

　　atmosphere,andthetwosystemsfunctiontogetherinastateofdynamic

　　equilibrium,thetransitionfromanoxygen-poortoanoxygen-richatmosphere

　　alsomusthaveoccurredduringthisperiod.

　　Alongwiththebuildupofmolecularoxygen（O2）cameaneventualincrease

　　inozone（O3）levelsintheatmosphere.（A）Becauseozonefiltersoutharmful

　　ultravioletradiation,thismadeitpossibleforlifetoflourishinshallow

　　waterandfinallyonland.（B）Thiscriticalstateintheevolutionofthe

　　atmospherewasreachedbetween1,100and542millionyearsago.（C）

　　Interestingly,thefossilrecordshowsanexplosionoflifeforms542million

　　yearsago.（D）

　　Oxygenhascontinuedtoplayakeyroleintheevolutionandformoflife.

　　Overthelast200millionyears,theconcentrationofoxygenhasrisenfrom10

　　percenttoasmuchas25percentoftheatmosphere,beforesettling（probably

　　notpermanently）atitscurrentvalueof21percent.Thisincreasehasbenefited

　　mammals,whicharevoraciousoxygenconsumers.Notonlydowerequireoxygento

　　fuelourhigh-energy,warm-bloodedmetabolism,ouruniquereproductivesystem

　　demandsevenmore.Anexpectantmother’sused（venous）bloodmuststillhave

　　enoughoxygeninittodiffusethroughtheplacentaintoherunbornchild’s

　　bloodstream.Itwouldbeverydifficultforanymammalspeciestosurviveinan

　　atmosphereofonly10percentoxygen.

　　Geologistscannotyetbecertainwhytheatmosphericoxygenlevels

　　increased,buttheyhaveahypothesis.First,photosynthesisisonlyonepartof

　　theoxygencycle.Thecycleiscompletedbydecomposition,inwhichorganic

　　carboncombineswithoxygenandformscarbondioxide.Butiforganicmatteris

　　buriedassedimentbeforeitfullydecomposes,itscarbonisnolongeravailable

　　toreactwiththefreeoxygen.Thustherewillbeanetaccumulationofcarbon

　　insedimentsandofoxygenintheatmosphere.

　　1.Inparagraph1,whydoestheauthorstatethatEarthhaslessnitrogen,

　　argon,hydrogen,andheliumthantheSun

　　A）Toarguethattheseelementswereoncepartofanearlyatmosphere,which

　　disappeared.

　　B）TosuggestthattheseelementsweredrawnintotheSun’satmosphere.

　　C）ToprovideevidencethatEarth’soriginalatmospherecameprimarilyfrom

　　meteorites.

　　D）TosupporttheclaimthatEarth’satmospherewouldhavechangedevenmore

　　ifithadcontainedmorevolatileelements.

　　2.Theword“constituent”inthepassageisclosestinmeaningto

　　A）source

　　B）equivalent

　　C）component

　　D）product

　　3.Accordingtoparagraph2,thehistoryofvolcanicoutgassingcannot

　　explainwhichofthefollowing

　　A）Thelackofoxygenintheatmosphere4billionyearsago.

　　B）Theamountofwatervaporintheatmospheretoday.

　　C）Theproportionsofnitrogenandcarbondioxideintheatmosphere

　　today.

　　D）Thepresentabundanceofoxygenintheatmosphere.

　　4.Paragraph3suggestswhichofthefollowingabouttheprocessof

　　photodissociation

　　A）Itismorecommontodaythanitwasintheearlyhistoryofthe

　　atmosphere.

　　B）Itisresponsibleforonlyasmallamountoftheoxygenintheatmosphere

　　C）Itremovestraceamountsofoxygenfromtheatmosphere.

　　D）Itproducesmorefreeoxygenthanphotosynthesisdoes.

　　5.Accordingtoparagraph4,whatcanbelearnedfromthetypeofiron

　　compoundsinseafloorrocks

　　A）Howtheprocessofphotosynthesishaschangedovertime

　　B）Thelevelofoxygeninthewateratacertaintimeinhistory

　　C）Howlevelsofironinoceanwaterdecreasedovertime

　　D）Theoverallmineralcontentoftheoceanwater

　　6.Theword“predominate”inthepassageisclosestinmeaningto

　　A）areinthemajority

　　B）arepresent

　　C）areincreasing

　　D）firstappear

　　7.Accordingtoparagraph4,banded-ironformationsarefoundinwhatkind

　　ofrocks

　　A）Thosethataremorethan2.5billionyearsold.

　　B）Thosethatdonotcontainoxidizedcompounds.

　　C）Thosethatarefromatransitionalperiodintermsofoxygen

　　richness.

　　D）Thosethatarelessthan1.8billionyearsold.

　　8.Whichofthesentencesbelowbestexpressestheessentialinformationin

　　thehighlightedsentenceinthepassageIncorrectchoiceschangethemeaningin

　　importantwaysorleaveoutessentialinformation.

　　A）Sincetheoceansandtheatmospherefunctiontogether,theatmosphere

　　musthavebecomeoxygenrichduringthisperiod.

　　B）Becauseoceanwaterisinconstantcontactwiththeatmosphere,thetwo

　　systemsmaintainadynamicequilibrium.

　　C）Thetransitiontoanoxygen-richatmospherecouldnothavehappened

　　withoutconstantcontactwiththeoceans.

　　D）Muchoftheoxygenintheoceansmusthavebeenpulledoutofthe

　　atmosphereduringthisperiod.

　　9.Accordingtoparagraph5,whichofthefollowinghappenedsometime

　　between1,100and542millionyearsago

　　A）Asuddenexplosionoflifeformsonlandoccurredtogetherwithasharp

　　declineoflifeinthewater.

　　B）Ultravioletradiationbecamemoreharmfultolivingorganisms.

　　C）Molecularoxygenlevelsintheatmospherestabilized,andozonelevels

　　begantorise.

　　D）Ozonereducedultravioletradiationtoalevelacceptableforlifeon

　　land.

　　10.Accordingtoparagraph6,whichofthefollowingisNOTtrueofthe

　　connectionbetweenmammalsandoxygen

　　A）Mammalsareabletosurviveonlybecauseoxygenlevelsarerelatively

　　high.

　　B）Mammalsfirstemergedwhenatmosphericoxygenlevelsreached10

　　percent.

　　C）Amammal’sunbornchildreceivesoxygenthroughthemother’s

　　placenta.

　　D）Mammalsusealotofoxygenpartlybecausetheyarewarm-blooded.

　　11.Theword“gradual”inthepassageisclosestinmeaningto

　　A）crucial

　　B）original

　　C）beneficial

　　D）slow

　　12.Theword“diffuse”inthepassageisclosestinmeaningto

　　A）spread

　　B）break

　　C）squeeze

　　D）speed

　　13.Lookatthefoursquaresthatindicatewherethefollowingsentence

　　couldbeaddedtothepassage.

　　Thetimingstronglysuggeststhatatmosphericchangeswereresponsiblefor

　　thissuddenincreaseinnewlife.

　　Wherewouldthesentencebestfit

　　14.Directions:

Anintroductorysentenceforabriefsummaryofthepassage

　　isprovidedbelow.CompletethesummarybyselectingtheTHREEanswerchoices

　　thatexpressthemostimportantideasinthepassage.Someanswerchoicesdonot

　　belonginthesummarybecausetheyexpressideasthatarenotpresentedinthe

　　passageorareminorideasinthepassage.

　　A）Overthelast4billionyears,outgassingdestroyedEarth’sprimary

　　atmosphereofvolatileelementsandreplaceditwithnonvolatilematerials

　　includingcarbondioxide.

　　B）ThesmallamountofoxygeninEarth’searlyatmospherewasdueto

　　photodissociationand,later,photosynthesiscreatedfreeoxygen.

　　C）Mammalscouldnothavesurvivedwithoutanoxygen-richatmosphere,and

　　land-basedlifewouldnotbepossiblewithouttheozonelayertofiltersolar

　　radiation.

　　D）Whenoxygenlevelsintheoceanwaterreachedacriticallevelabout542

　　millionyearsago,lifeemergedintheoceans,asshownbysedimenta