Peopleandthesourcesofairpollutionarefoundinthesameplaces.Thismeansthatcitieswithlargepopulationshavethebiggestproblemofdirtyair.Airpollutioniscausedbymanydifferentthings.Amajorsourceofairpollutionisthegasfumesfromcars.Statisticsshowthat93percentofallautotripsarewithincities.Anothermajorsourceofdirtyairistheburningofcoalandoilforenergy.Thisenergyisneededtomakeelectricity.Ofcoursemuchmoreelectricityisusedinthecitythaninthecountry.

Ontheaveragewethrowawaymoretrashandgarbagethantheyearbefore.Theburningofgarbagecontributestoairpollution.Manymajorindustriesarealsoresponsibleforthedirtyairinthearoundcities.Thefumesfromironsteelchemicalandpetroleumproductionaddparticlestotheair.

Theeffectsofairpollutionrangefrommildheadachestodeath.Thelevelsofpollutionfoundinheavyfortrafficmaycauseheadachesforlossofclearvision.Wherevercoalandoilareusedforfuelfumesmaykilltreesandplantsandcausemetaltocorrode.Insomeofthelargercitiesthesefumesendangertheliveofhumanbeingsbycontributingtolungdiseasesandcausingearlydeath.

67.Thekeypointofthepassageisthat_________.

A.thecauseofairpollutionispeople

B.thecausesandtheeffectsofairpollutionarebothfoundincities

C.theeffectsofairpollutionrangefromheadachestodeath

D.airpollutioniscausedbydirtyair

68.Whatisthepurposeofthispassage?

A.topersuadepeopletostoppollutingtheair

B.totellthecausesofairpollution

C.totellwhycitiesarebadplacestolive

D.todescribewhycitiesarebadplacestolive

69.Whyisairpollutionmoregraveinthecitythaninthecountry?

A.Becausetherelargerpopulationsincities.

B.Becausetheairinthecityisdirty.

C.Becausetherearemorecarsinthecity.

D.Becausetherearemuchmoregasfumesfromcarsandburningofcoalandoilforenergyinthecitythaninthecountry.

70.Thefumesfromcoalandoilmaycausemetal______.

A.toturnblack B.tobecomerusty C.tocorrode D.totwist

71.Inlargercitiesthe