大学英语四级考试试题真题-(9).doc

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大学英语四级考试试题真题-(9).doc

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大学英语四级考试试题真题

Passage?four

????Questions21to25arebasedonthefollowingpassage.

Leaddeposits,whichaccumulatedinsoilandsnowduringthe1960sand70s,wereprimarilytheresultofleadedgasolineemissionsoriginatingintheUnitedStates.InthetwentyyearsthattheCleanAirActhasmandatedunleadedgasuseintheUnitedStates,theleadaccumulationworld-widehasdecreasedsignificantly.

AstudypublishedrecentlyinthejournalNatureshowsthatair-borneleadedgasemissionsfromtheUnitedStatesweretheleadingcontributortothehighconcentrationofleadinthesnowinGreenland.ThenewstudyisaresultofthecontinuedresearchledbyDr.CharlesBoutron,anexpertontheimpactofheavymetalsontheenvironmentattheNationalCenterforScientificResearchinFrance.AstudybyDr.Boutronpublishedin1991showedthatleadlevelsinarctic(北极的)snowweredeclining.

Inhisnewstudy,Dr.BoutronfoundtheratiosofthedifferentformsofleadintheleadedgasolineusedintheUnitedStatesweredifferentfromtheratiosofEuropean,AsianandCanadiangasolinesandthusenabledscientiststodifferentiate(分区)theleadsources.ThedominantleadratiofoundinGreenlandsnowmatchedthatfoundingasolinefromtheUnitedStates.

InastudypublishedinthejournalAmbio,scientistsfoundthatleadlevelsinsoilintheNorth-easternUnitedStateshaddecreasedmarkedlysincetheintroductionofunleadedgasoline.

Manyscientistshadbelievedthattheleadwouldstayinsoilandsnowforalongerperiod.

TheauthorsoftheAmbiostudyexaminedsamplesoftheupperlayersofsoiltakenfromthesamesitesof30forestfloorsinNewEngland,NewYorkandPennsylvaniain1980andin1990.

Theforestenvironmentprocessedandredistributedtheleadfasterthanthescientistshadexpected.

Scientistssaybothstudiesdemonstratethatcertainpartsoftheecosystem(生态系统)respondrapidlytoreductionsinatmosphericpolluti

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大学英语四级考试试题真题-(9).doc