Questions61-65arebasedonthefollowingpassage.
Ausefuldefinitionofanairpollutantisacompoundaddeddirectlyorindirectlybyhumanstotheatmosphereinsuchquantitiesastoaffecthumans,animals,vegetation,ormaterialadversely(有害地..Airpollutionrequiresaveryflexibledefinitionthatpermitscontinuouschange.WhenthefirstairpollutionlawswereestablishedinEnglandinthefourteenthcentury,airpollutantswerelimitedtocompoundsthatcouldbeseenorsmelled—afarcry(悬殊的差别.fromtheextensivelistofharmfulsubstancesknowntoday.Astechnologyhasdevelopedandknowledgeofthehealthaspectsofvariouschemicalshasincreased,thelistofairpollutantshaslengthened.Inthefuture,evenwatervapor(水蒸气.mightbeconsideredanairpollutantundercertainconditions.
Manyofthemoreimportantairpollutants,suchassulfuroxides,carbonmonoxide,andnitrogenoxides,arefoundinnature.AstheEarthdeveloped,theconcentrations(浓度.ofthesepollutantswerealteredbyvariouschemicalreactions;theybecamecomponentsinbiogeochemical(生物地球化学的.cycles.Theseserveasanairpurificationschemebyallowingthecompoundstomovefromtheairtothewaterorsoil.Onaglobalbasis,nature'soutputofthesecompoundsdwarfsthatresultingfromhumanactivities.However,humanproductionusuallyoccursinalocalizedarea,suchasacity.
Inthislocalizedregion,humanoutputmaybedominantandmaytemporarilyoverloadthenaturalpurificationschemeofthecycles.Theresultisanincreasedconcentrationofnoxious(有害的.chemicalsintheair.Theconcentrationsatwhichtheadverseeffectsappearwillbegreaterthantheconcentrationsthatthepollutantswouldhaveintheabsenceofhumanactivities.Theactualconcentrationneednotbelargeforasubstancetobeapollutant;infactthenumericalvaluetellsuslittleuntilweknowhowmuchofanincreasethisrepresentsovertheconcentrationthatwouldoccurnaturallyinthearea.Forexample,sulfurdioxidehasdetectablehealtheffectsat0.08partspermillion(ppm.,whichisabout400timesitsnaturallevel.Carbonmonoxide,however,hasanaturallevelof0.1ppmandisnotusuallyapollutantuntilitslevelreachesabout15ppm.
61、Itcanbeinferredfromthefirstparagraphthat_________
A.watervaporisanairpollutantinlocalizedareas
B.thedefinitionofairpollutionwillcontinuetochange
C.asubstancebecomesanairpollutantonlyincities
D.mostairpollutantstodaycanbeseenorsmelled
62、Inwhatwaycannaturalpollutantsplayanimportantroleincontrollingairpollution?
A.Theyfunctionaspartofapurificationprocess.
B.Theyaredwarfedbythepollutantsproducedbyhumanactivities.
C.Theyarelessharmfultolivingbeingsthanareotherpollutants.
D.TheyhaveexistedsincetheEarthdeveloped.
63、Accordingtothepassage,thenumericalvalueOftheconcentrationlevelofasubstanceisonlyusefulif
A.theothersubstancesintheareaareknown
C.itcanbecalculatedquickly
D.thenaturallyoccurringlevelisalsoknown
64、Whichofthefollowingisbestsupportedbythepassage?
A.Toeffectivelycontrolpollution,localgovernmentshouldregularlyrevisetheairpollutionlaws.
B.Oneofthemostimportantstepsinpreservingnaturallandsistobetterenforceairpollutionlaws.
C.Scientistsshouldbeconsultedinordertoestablishuniformlimitsforallairpollutants.
D.Humanactivitieshavegreatimpactonairpollution.
65、Thepassagemainlydiscusses_________
A.theeconomicimpactonairpollution
B.howmuchdamageairpollutantscancause
C.whatconstitutesanairpollutant
D.thequantityofcompoundsaddedtotheatmosphere
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