题目内容：
Questionsarebasedonthefollowingpassage. EverypeoplehasitsownspecialwordsandexpressionsliketheAmericanexpression"onthewagon"．Itmeansap.ersonwhonolongerdrinkswhisky,beerorotherkindsofalcoholicdrinks．The"wagon"isawaterwagon．Inotherwords,adrinkerhasdecidedtochangefromalcoholtowater．AnOklahomanewspaperwroteaboutamanwhohadahistoryofheavydrinkingonAmerican'sFourthofJulyholiday．Thenewspapersaid,"webelievedthathisexperiencehadliftedhimontothewaterwagon,buthewasevendrunkerthenextFourthofJuly."
Whydopeopledecidetogoonthewagon?Well,usuallybecausetheyarealcoholics.
Theylosecontrolofthemselveswhentheydrink．Iftheytakeonealcoholicdrink,thentheywanttodrinkanother,andanotheruntilallthoughtisgone．Alcoholismisamajorsocialproblemincountriesthroughouttheworld．TheUnitedStatesisnoexception.
ExpertsbelievethatseveralmillionAmericansarealcoholicstoday．Theirhealthcareneeds,caraccidents,reducedjobperformanceandotherproblemscostthousandsofmillionsofdollarseveryyear．Intheearly1900s,manyAmericansjoinedinacampaigntomakealcoholillegalintheUnitedStates．In1920anamendmentwasaddedtotheConstitution．Itmadedrinkingorsellingalcoholacrime．Butthenewlawdidnotstoppeoplefromdrinking．Criminalsproducedorimportedalcoholillegallyandsolditalmosteverywhere.
AfterafewyearsAmericansrealizedtheso-calledProhibitionLawdidnotwork．Soin1933anotherconstitutionalchangewasmade,cancelingtheamendmentthatmadealcoholillegal.
InrecentyearsAmericanshavetakenotherstepstodealwiththeproblemofalcohol.
Oneimportantactionhasbeentoincreasethepunishmentforpeoplewhodrivetheir
40automobilesafterdrinkingalcohol．Manystatesnowconsiderthistobeaseriouscrime.Anotherimportantstephasbeentoexplaintochildrenandstudentsthedangersofalcohol.
Manyschoolsnowofferspecialprogramstoteachyoungpeopleabouttheproblem．Doctorshavebeenworkingtodevelopnewdrugsandtreatmentprogramstohelpalcoholics.Alcoholicsthemselvesaredealingwiththeproblemofalcoholism．Manyofthemhave"goneonthewagon"andstoppeddrinking．TheyhavejoinedtheAlcoholicsAnonymousgroupmeetingregularlytodiscusstheirproblemsandgiveeachothersupport．"Goingonthewagon"isnottheonlywaytoconqueralcoholism,butmostalcoholicsayitisthebestway.
Theymustgiveupalcoholcompletelysotheirownbodyandspiritcansurvive.
Themainideaofthepassageis__________. A．toexplainthephrase"onthewagon"
B．todiscusswhypeoplegoonthewagon
C．todemonstratethenegativeeffectofalcoholism
D．tocommentonthemovementof"goonthewagon"
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