AgricultureisthenumberoneindustryintheUnitedStatesandagriculturalproductsarethecountry'sleadingexport.AmericanfarmersmanagetofeednotonlythetotalpopulationoftheUnitedStates,butalsomillionsofotherpeoplethroughouttherestoftheworld.Cornandsoybeanexportsaloneaccountforapproximately75percentoftheamountsoldinworldmarkets.
Thisproductivity,however,hasitsprice.Intensivecultivationexposestheearthtothedamagingforcesofnature.Everyyearwindandwaterremovetonsofrichsoilfromthenation'scroplands.
Eachfieldiscoveredbyalimitedamountoftopsoil,theupperlayerofearthwhichisrichestinthenutrientsandmineralsnecessaryforgrowingcrops.EversincethefirstfarmersarrivedintheMidwestalmost200yearsago，cultivationand,consequently,erosionhavebeendecreasingthesupplyoftopsoil.Inthe1830s,nearlytwofeetofrich,blacktopsoilcoveredtheMidwest.Todaytheaveragedepthisonlyeightinches,andeverydecadeanotherinchisblownorwashedaway.Thiserosionissteadilydecreasingtheproductivityofvaluablecropland.AUnitedStatesAgriculturalDepartmentsurveystatesthatiferosioncontinuesatitspresentrate,cornandsoybeanyieldsintheMidwestmaydropasmuchas30percentoverthenext50years.
Sofar,farmershavebeenabletocompensateforthelossoffertiletopsoilbyapplyingmorechemicalfertilizerstotheirfields;however,whilethispracticehasincreasedcropyields,ithasbeendevastatingforecology.Agriculturehasbecomeoneofthebiggestpollutersofthenation'spreciouswatersupply.Rivers,lakes,andundergroundreservesofwaterarebeingfilledinandpoisonedbysoilandchemicalscarriedbydrainagefromerodingfields.Furthermore,fertilizersonlyreplenishthesoilstheydonotpreventitsloss.
26.Thelastsentenceinthefirstparagraphgivesanexampletoshow_______.
A.thatAmericanfarmersmanagetofeedthetotalpopulationoftheU.S.
B.theleadingpositionoftheU.S.farmingintheworld
C.howimportantAmericanpeopleconsidertheirfarming
D.thatmanypeopleintheworldrelyontheexportoftheagriculturalproductsoftheU.S.A.
27.Inordertocompensateforthelossoffertiletopsoil,farmershavebeen_______.
A.plantinglesscornandsoybeanB.puttingfertilizersontheirfields
C.preventingsoilerosionD.decreasingthesupplyoftopsoil
28.Atthepresentrate,approximatelyhowmanyyearslatertheblacktopsoilnowcoveringtheMidwestwillcompletelybeblownorwashedaway?
A.120yearslater..B.80yearslater.
C.50yearslater.D.100yearslater.
29."Thispractice"inParagraph4refersto_______.
A.thatfarmershaveloweredtheyieldofcornandsoybean
B.thatfarmershaveexpandedcroplands
C.thatfarmershaveappliedmorechemicalfertilizers
D.thatthetopsoilhasbeendecreasedgreatly
30.AllofthefollowingarestatementsaboutthedisadvantagesoffertilizersEXCEPTthat________.
A.theyreplenishthesoilB.theydonotpreventthelossofsoil
C.theyaredestroyingtheecologyD.theypollutethenation'swatersupply
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