题目内容：
Questions37to40arebasedonthefollowingpassage：
Securityandcommodityexchangesaretradingpostswherepeoplemeetwhowishtobuyorsell．Theexchangesthemselvesdonotrading；theymerelyprovideaplacewhereprospectivebuyersandsellerscanmeetandconducttheirbusiness. WallStreet，althoughthebestknown，isnottheonlyhomeofexchangesintheUnitedStates.TherearecottonexchangesinNewOrleansandChicago；theMercantileExchange，whichdealsinmanyfarmproducts，inChicago；andgrainexchangesinmanyofthelargecitiesoftheMidwest.homeexchanges，liketheChicagoBoardofTrade，providemarketservicesforseveralkindsofproducts.Thesetradingpostswhereproductsmaybeboughtorsoldarecalledcommodityexchanges. Thesecurityexchanges，ontheotherhand,aremeetingplaceswherestocksandbondsaretraded．Likethecommodityexchanges，theyhelpservetheeconomiclifeofthecountry.Butwhentheiroperationsgetoutofhand，theymaybecomeverydangerous．Inl929，thesecurityexchanges，orstockmarket，contributedtoacrashasudden，sharpdeclineinthevalueofsecurities.Manypeoplelostfortunes；manycorporationswerebankrupted；manyworkerslosttheirjobs.TheCrashofl929hasbeenattributedtomanycauses，amongthemwildandunwisespeculationbymanypeopleanddishonestpracticesonthepartofsomebusinessmenandofsomemembersoftheexchanges. Today，however，investingthroughsecurityexchangesandtradingoncommodityexchangeshasbeenmadesaferbyregulationssetupbytheexchangesthemselvesandbyregulationsoftheUnitedStatesgovernment，Inl922，thegovernmentinstitutedtheCommodityExchangeCommissionwhichoperatedthroughtheDepartmentofAgriculture；andinl934，theSecuritiesandExchangesCommission，toprotectinvestorsandthepublicagainstdishonestpracticesontheexchanges. 37．Securityandcommodityexchangesaremeetingplacesforbuyersandsellersof__________. A.stocks
B.grain
C.securities
D.allofthese
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答案解析：