题目内容：
Questionsarebasedonthefollowingpassage．
Someoftheworld’smostsignificantproblemsneverhitheadlines．Oneexamplecomesfromagriculture．Foodriotsandhungermakenews．Butthetrendlyingbehindthesemattersisrarelytalkedabout．Thisisthedeclineinthegrowthinyieldsofsomeoftheworld’smajorcrops．AnewstudybytheUniversityofMinnesotaandMcGillUniversityinMontreallooksatwhere．andhowfar．thisdeclineisoccurring．
Theauthorstakeavastnumberofdatapointsforthefourmostimportantcrops：rice，wheat，cornandsoyabeans(大豆)．Theyfindthatonbetween24％and39％ofallharvestedareas，theimprovementinyieldsthattookplacebeforethel980ssloweddowninthel990sand2000s．Therearetwoworryingfeaturesoftheslowdown．Oneisthatithasbeenparticularlysharpintheworld’smostpopulous(人口多的)countries，IndiaandChina．Theirabilitytofeedthemselveshasbeenanimportantsourceofrelativestabilitybothwithinthecountriesandonworldfoodmarkets．
Thatself-sufficiencycannotbetakenforgrantedifyieldscontinuetoslowdownorreverse．Second，yieldgrowthhasbeenlowerinwheatandricethanincollandsoyabeans．Thisisproblematicbecausewheatandricearemoreimportantasfoods，accountingforaroundhalfofallcaloriesconsumed．Cornandsoyabeansaremoreimportantasfeedgrains．Theauthorsnotethat“wehavepreferentiallyfocusedourcropimprovementeffortsonfeedinganimalsandcarsratherthanoncropsthatfeedpeopleandarethebasisoffoodsecurityinmuchoftheworld．”
Thereportqualifiesthemoreoptimisticfindingsofanothernewpaperwhichsuggeststhattheworldwillnothavetodigupalotmorelandforfarminginordertofeed9billionpeoplein2050，astheFoodandAgricultureOrganisationhasargued．Instead，itsays，thankstoslowingpopulationgrowth，landcurrentlyploughedupforcropsmightbeabletorevert(回返)toforestorwilderness．Thiscouldhappen．Thetroubleisthattheforecastassumescontinuedimprovementsinyieldswhichmaynotactuallyhappen
Whatdoestheauthortrytodrawattentionto? A．Foodriotsandhungerintheworld．
B．Newsheadlinesintheleadingmedia．
C．Thedeclineofthegrainyieldgrowth．
D．Thefoodsupplyinpopulouscountries．
参考答案：
答案解析：