E
Alltoooften,achoicethatseemssustainable（可持续的）turnsoutoncloserexaminationtobeproblematic.Probablythebestexampleistherushtoproduceethanol（乙醇）forfuelfromcorn.Cornisarenewableresource—youcanharvestitandgrowmore,almostlimitlessly.Soreplacinggaswithcornethanolseemslikeagreatidea.
Onemightgetabitmoreenergyoutoftheethanolthanthatusedtomakeit,whichcouldstillmakeethanolmoresustainablethangasgenerally,butthat’snottheendoftheproblem.Usingcorntomakeethanolmeanslesscornislefttofeedanimalsandpeople,whichdrivesupthecostoffood.Thatr esultleadstoturningthefallowland—including,insomecases,rainforestinplacessuchasBrazil—intofarmland,whichinturngivesofflotsofcarbondioxide（CO2）intotheair.Finally,overmanyyears,theenergybenefitfromburningethanolwouldmakeupfortheforestloss.Butbythen,climatechangewouldhaveprogressedsofarthatitmightnothelp.
Youcannotr eallydeclareanypractice“sustainable”untilyouhavedoneacompletelift-cycleanalysisofitsenvironmental（环境的）costs.Eventhen,technologyandpublicpolicykeepdeveloping,andthatd evelopmentcanleadtounforeseenandundesiredresults.Theadmirablegoaloflivingsustainablerequiresplentyofthoughtonanongoingbasis.
57.Whatmightd irectlycausethelossoftheforestaccordingtothetext?
A.Thegrowingdemandforenergytomakeethanol.
B.Theincreasingcarbondioxideintheair.
C.Thegreaterneedforfarmland.
D.Thebigchangeinweather.
正确答案：C
