托福真题24.docx

托福真题24.docx

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托福真题24

LakeWater（TPO24-1）

Wheredoesthewaterinalakecomefrom,andhowdoeswaterleaveit?

Waterentersalakefrominflowingrivers,fromunderwaterseepsandsprings,fromoverlandflowoffthesurroundingland,andfromrainfallingdirectlyonthelakesurface.Waterleavesalakeviaoutflowingrivers,bysoakingintothebedofthelake,andbyevaporation.Somuchisobvious.

Thequestionsbecomemorecomplicatedwhenactualvolumesofwaterareconsidered:

howmuchwatereiStersandleavesbyeachroute?

Discoveringtheinputsandoutputsofriversisamatterofmeasuringthedischargesofeveryinflowingandoutflowingstreamandriver.Thenexchangeswiththeatmospherearecalculatedbyfindingthedifferencebetweenthegainsfromrain,asmeasured（ratherroughly）byraingauges,andthelossesbyevaporation,measuredwithmodelsthatcorrectfortheothersourcesofwaterloss.Forthemajorityoflakes,certainlythosesurroundedbyforests,inputfromoverlandflowistoosmalltohaveanoticeableeffect.Changesinlakelevelnotexplainedbyriverflowsplusexchangeswiththeatmospheremustbeduetothenetdifferencebetweenwhatseepsintothelakefromthegroundwaterandwhatleaksintothegroundwater.Notetheword"net":

measuringtheactualamountsofgroundwaterseepageintothelakeandoutofthelakeisamuchmorecomplicatedmatterthanmerelyinferringtheirdifference.

Onceallthisinformationhasbeengathered,itbecomespossibletojudgewhetheralake’sflowismainlyduetoitssurfaceinputsandoutputsortoitsundergroundinputsandoutputs.[■]Iftheformeraregreater,thelakeisasurface-water-dominatedlake;ifthelatter,itisaseepage-dominatedlake.[■]Occasionally,commonsensetellsyouwhichofthesetwopossibilitiesapplies.[■]Forexample,apondinhillycountrythatmaintainsasteadywaterlevelallthroughadrysummerinspiteofhavingnostreamsflowingintoitmustobviouslybeseepagedominated.Conversely,apondwithastreamflowinginoneendandouttheother,whichdriesupwhenthestreamdriesup,isclearlysurfacewaterdominated.[■]

Bywhatevermeans,alakeisconstantlygainingwaterandlosingwater:

itswaterdoesnotjustsitthere,or,anyway,notforlong.Thisraisesthematterofalake’sresidencetime.Theresidencetimeistheaveragelengthoftimethatanyparticularmoleculeofwaterremainsinthelake,anditiscalculatedbydividingthevolumeofwaterinthelakebytherateatwhichwaterleavesthelake.Theresidencetimeisanaverage;thetimespentinthelakebyagivenmolecule（ifwecouldfollowitsfate）woulddependontherouteittook:

itmightflowthroughaspartofthefastest,mostdirectcurrent,oritmightcircleinabackwaterforanindefinitelylongtime.

Residencetimesvaryenormously.Theyrangefromafewdaysforsmalllakesuptoseveralhundredyearsforlargeones;LakeTahoe,inCalifornia,hasaresidencetimeof700years.TheresidencetimesfortheGreatLakesofNorthAmerica,namely,LakesSuperior,Michigan,Huron,Erie,andOntario,are,respectively,190,100,22,2.5,and6years.LakeErie’sisthelowest:

althoughitsareaislargerthanLakeOntario’s,itsvolumeislessthanone-thirdasgreatbecauseitissoshallow-lessthen20metersonaverage.

Agivenlake’sresidencetimeisbynomeansafixedquantity.Itdependsontherateatwhichwaterentersthelake,andthatdependsontherainfallandtheevaporationrate.Climaticchange（theresultofglobalwarming?

）iscjamaticallyaffectingtheresidencetimesofsomelakesinnorthwesternOntario,Canada.Intheperiod1970to1986,rainfallintheareadecreasedfrom1,000millimetersto650millimetersperannum,whileabove-averagetemperaturesspeededuptheevapotranspirationrate（therateatwhichwaterislosttotheatmospherethroughevaporationandtheprocessesofplantlife）.Theresulthasbeenthattheresidencetimeofoneofthelakesincreasedfrom5to18yearsduringthestudyperiod.Theslowingdownofwaterrenewalleadstoachainoffurtherconsequences;itcausesdissolvedchemicalstobecomeincreasinglyconcentrated,andthis,inturn,hasamarkedeffectonalllivingthingsinthelake.

1.Thephrase"Somuch"inthepassagerefersto

Athenegativeeffectsofoverlandflow,rain,andevaporationonriverwaterlevels

Bwaterthatalakelosestooutflowingrivers,tothelakebed,andtoevaporation

Ctheimportanceofriverstothemaintenanceoflakewaterlevels

Dtheinformationgivenaboutwaysthatwatercanenterorexitalake

2.Theword"gains"inthepassageisclosestinmeaningto

Aresults

Bincreases

Cresources

Dsavings

3.Whichofthefollowingcanbeinferredfromparagraph2aboutthemovementofwaterintoalake?

AHeavyrainaccountsformostofthewaterthatentersintolakes.

BRainfallreplacesapproximatelytheamountofwaterlostthroughevaporation.

COverlandflowintolakesisreducedbythepresenceofforests.

DSeepagehasasmallereffectonwaterlevelthananyotherinput.

4.Whydoestheauthorusethephrase"Notetheword"net""inthepassage?

AToemphasizetheimpactofseepageonwaterlevels

BTopointoutthatseepageiscalculateddifferentlyfromriverflowsandatmosphericexchanges

CTocomparethedifferentmethodsofcalculatingseepage

DToemphasizethedifficultyofobtainingspecificvaluesforseepageinputsandoutputs

5.Theword"Conversely"meaningto

Aontheotherhand

Binthesameway

Cinotherwords

Donaverage

6.Accordingtoparagraph3,whichofthefollowingbestdescribesaseepage-dominatedlake?

AAlakethatisfedbystreamsbutstillhasfluctuatingwaterlevels

BAlakewithaconstantwaterlevelthathasnostreamsorriversasinputs

CAlakewithastreamflowingintoitandastreamflowingoutofit

DAlakethathassurfaceandundergroundinputsbutloseswaterduringdryseasons

7.Itcanbeinferredfromparagraph4thatthelengthoftimeagivenmoleculeofwaterremainsinalake

Adependsentirelyupontheaveragespeedofalake'scurrents

Bcanbemeasuredbythevolumeofthelakealone

Ccanbegreaterorlesserthantheresidencetime

Dissimilartothelengthoftimeallothermoleculesremaininthatlake

8.Accordingtoparagraph5,LakeErie'sresidencetimeislowerthanLakeOntario'sforwhichofthefollowingreasons?

ALakeEriehasalargerareathanLakeOntario.

BLakeOntarioisshallowerthanLakeErie.

CLakeOntariohasagreatervolumethanLakeErie.

DLakeEriereceiveslessrainfallthanLakeOntario.

9.WhydoestheauthordiscusstheGreatLakesinparagraph5?

ATodemonstratetheextenttowhichresidencetimesvaryfromlaketolake

BToillustratehowresidencetimesarecalculatedforspecificlakes

CToarguethattheresidencetimeofalakeincreaseswitharea

DToemphasizethatLakeTahoe'sresidencetimeisunusuallylong

10.Theword"further"inthepassageisclosestinmeaning

Aexpected

Badditional

Cserious

Dunfortunate

11.Accordingtoparagraph6,whichofthefollowingexplainstheincreaseinresidencetimeofsomelakesofnorthwesternOntario?

ATheamountofwaterflowingintothelakeshasincreased.

BTherateofevaporationhasdecreasedmoresharplythantheamountofrainfall.

CTherenewalofthelakes'waterhasslowedduetochangesinclimate.

DPlantshaverequiredlesswaterfromthelakes.

12.Accordingtoparagraph6,residencetimeisaffectedbyallofthefollowingEXCEPT

Aamountofrainfall

Brateofevaporation

Ctemperatureofsurroundingair

Dconcentrationofchemicalsinlakewater

13.Lookatthefoursquares[■]thatindicatewherethefollowingsentencecouldbeaddedtothepassage.

Ofcourse,alakemaybeneithersurface-water-norseepage-dominatedif,forexample,itsinputsarepredominantlysurfaceanditsoutputsarepredominantlyseepage.

Wherewouldthesentencebestfit?

Clickonasquaretoaddthesentencetothepassage.

14.Directions:

Anintroductorysentenceforabriefsummaryofthepassageisprovidedbelow.CompletethesummarybyselectingtheTHREEanswerchoicesthatexpressthemostimportantideasinthepassage.Somesentencesdonotbelonginthesummarybecausetheyexpressideasthatarenotpresentedinthepassageorareminorideasinthepassage.Thisquestionisworth2points.

Waterenters,remains,andeventuallyleavesalakeinavarietyofways.

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AnswerChoices

ABymeasuringthewaterquantitiesateachofalake'sinputsandoutputs,itcanbedeterminedwhetherwaterentersthelakemainlyfromsurfaceorgroundwatersources.

BChangesinlakelevelandvolumearecausedprincipallybytheamountofevaporationofwaterintotheatmosphere.

CItissometimespossibletodecidewhetheralakeissurfacewaterdominatedorseepagedominatedbysimpleobservationatdifferentseasons.

DTheaverageperiodoftimethatmoleculesofwaterspendinalake—theresidencetime—variesfromlaketolakeandovertimewithinaparticularlake.

ETheresidencetimesofsurface-water-dominatedlakesareusuallylongerthanthoseofseepage-dominatedlakes.

FTheresidencetimeofalakefrequentlydependsonthekindsoforganismstobefoundinthelake.

BreathingDuringSleep（TPO24-2）

Ofallthephysiologicaldifferencesinhumansleepcomparedwithwakefulnessthathavebeendiscoveredinthelastdecade,changesinrespiratorycontrolaremostdramatic.Notonlyaretheredifferencesinthelevelofthefunctioningofrespiratorysystems,thereareevenchangesinhowtheyfunction.Movementsoftheribcageforbreathingarereducedduringsleep,makingthecontraction