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：第五段是作者对于以上两种观点的总结:我们也不确定会发生哪种情况。A选项的thetwotendencies指代前面谈到的两种观点在词汇结构上一致;而perhaps同样反映了作者的不确定,与前面的wedonotknowforsure相对应,因此选项A为此处应填句子