Questions21to25arebasedonthefollowingpassage.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.TheauthorsoftheAmbiostudyexaminedsamplesoftheupperlayersofsoiltakenfromthesamesitesof30forestfloorsinNewEngland,NewYorkandPennsylvaniain1980andin1990.Theforestenvironmentprocessedandredistributedtheleadfasterthanthescientistshadexpected.Scientistssaybothstudiesdemonstratethatcertainpartsoftheecosystem(生态系统)respondrapidlytoreductionsinatmosphericpollution,butthatthesefindingsshouldnotbeusedasalicensetopollute.Itcanbeinferredfromthelastparagraphthatscientists（）

A. arepuzzledbythemysteryofforestpollution
B. feelrelievedbytheuseofunleadedgasoline
C. stillconsiderleadpollutionaproblem
D. lacksufficientmeanstocombatleadpollution