题目内容：
Questionsarebasedonthefollowingpassage. Leaddeposits,whichaccumulatedinsoilandsnowduringthe1960’sand70’s,wereprimarilytheresultofleadedgasolineemissionsoriginatingintheUnitedStates.InthetwentyyearsthattheCleanAirActhasmandatedunleadedgasuseintheUnitedStates,theleadaccumulationworldwidehasdecreasedsignificantly.
AstudypublishedrecentlyinthejournalNatureshowsthatair-borneleadedgasemissionsfromtheUnitedStatesweretheleadingcontributortothehighconcentrationofleadinthesnowinGreenland.ThenewstudyisaresultofthecontinuedresearchledbyDr.CharlesBoutron,anexpertontheimpactofheavymetalsontheenvironmentattheNationalCenterforScientificResearchinFrance.AstudybyDr.Boutronpublishedin1991showedthatleadlevelsinarctic（北极的）snowweredeclining.
Inhisnewstudy,Dr.BoutronfoundtheratiosofthedifferentformsofleadintheleadedgasolineusedintheUnitedStatesweredifferentfromtheratiosofEuropean,AsianandCanadiangasolinesandthusenabledscientiststodifferentiate（区分）theleadsources.ThedominantleadratiofoundinGreenlandsnowmatchedthatfoundingasolinefromtheUnitedStates.
InastudypublishedinthejournalAmbio,scientistsfoundthatleadlevelsinsoilintheNortheasternUnitedStateshaddecreasedmarkedlysincetheintroductionofunleadedgasoline.
Manyscientistshadbelievedthattheleadwouldstayinsoilandsnowforalongerperiod.
TheauthorsoftheAmbiostudyexaminedsamplesoftheupperlayersofsoiltakenfromthesamesitesof30forestf
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