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题目内容
（国家开放大学开放英语）
Thereasonsforthechangesarecomplex.Itisclearthatpollutionoftheairandwaterbyfactoriesisaffectingtheenvironment.Treesandothervegetationdie,andsosoiliseroded.themoretreesthatdie,thelesswaterisheldintheearth.theneedforwoodforfuelandbuildingsleadstofurtherdeforestation,whichcausesfurthersoilerosion.Localfarmersfinditmoreandmoredifficulttomakealiving.theyendupusinginefficientfarmingmethods,whichagainleadstosoilerosion.
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