Air PollutionThere was one thought that air pollution affected only th

AirPollution

Therewasonethoughtthatairpollutionaffectedonlytheareasimmediatelyaroundlargecitieswithfactoriesandheavyautomobiletr affic.____1____

Onseveraloccasionsoverthepastd ecade,aheavycloudofairpollutionhascoveredtheeastoftheUnitedStatesandbroughthealthwarningsinruralareasawayfromanymajorconcentr ationofmanufacturingandautomobiletr affic.Infact,theveryclimateoftheentireearthmaybeinfectedbyairpollution

Somescientistsconsiderthattheincreasingconcentr ationofcarbondioxide((二氧化碳)intheairresultingfromtheburningofcoalandoiliscreatinga"greenhouseeffect".Itconservesheatr eflectedfromtheearthandraisedtheworld’saveragetemperature.___2___ThusmuchofthepolaricecapwillmeltandcitiessuchasNewYork,Boston,Miami,andNewOrleanswillbeinwater.

___3___Thisisaresultthatwouldbeequallydisastr ous.Adropofjustafewdegreescouldcreatesomethingclosebetoanewiceage,andwouldmakeagriculturedifficultorimpossibleinmanyofourtopfarmingareas.

Todaywedonotknowforsurethateitheroftheseconditionswillhappen.___4___Drivenbyeconomicprofit,peopleneglectbythedamageonourenvironmentcausedbythe"advancedcivilization".___5___Butisitr eallyworthwhile?

Perhaps,ifweareluckyenough,thetwotendencieswilloffset(抵消)eachotherandtheworld'stemperaturewillstayaboutthesameasitisnow.

Ifthisviewiscorrect,theworld'stemperatureisraisedonlyafewdegrees.

Maybetheairpollutionisthepricethehumanbeingshavetopayfortheirdevelopment

Atpresent,werealizethatalthoughthesearetheareaswiththeworstairpollution,theproblemisliterallyworldwide

Lately,globalwarmingisbecomingaseriousproblem

Anotherview,lesswidelyheld,isthatincreasingparticularmatterintheatmosphere(大气)isblockingsunlightandloweringtheearth'stemperature.

答案及解析：文章开篇讲述了人们对空气污染这一问题的认识。第一句话讲迷过去人们的想法是“空气污染曾一度被认为只影响拥有工厂和交通拥堵的大城市的周边地区。”浏览备选答案,不难发现选项D中的Atpresent作为时间块语,在结构上有着承上启下的作用,讲述现在人们对于污染的认识。因此,D选项为空格处应填D