题目内容：
Questionsarebasedonthefollowingpassage.
Newresearchsuggeststhatspeakingmorethanonelanguagemaydelaydifferentkindsofdementia(痴呆),thatisthelostofmentalability.Infact,researcherssay,speakingtwolanguagesappearstobemoreimportantthanthelevelofeducationindefendingagainstdementias.
AstudyinIndiaexaminedtheeffectofknowingmorethanonelanguageindelayingthefirstsignsofseveraldisorders.Researchersstudiednearly650peoplewhoseaverageagewas66.240ofthosestudiedsufferedfromAlzheimer's(老年痴呆症),themostcommonformofmentaldecline.391ofthesubjectsspoketwoormorelanguages.Investigatorsfoundthedementiasbeganaboutfour-and-a-halfyearslaterinthosewhospoketwolanguagescomparedtothosewhospokeonlyonelanguage.
ThomasBakhelpedtoorganisethestudy.HeiswiththeCenterofCognitiveAgingattheUniversityofEdinburghinScotland.Hesuggeststhatindividualswhospeakmorethanonelanguagetraintheirbrainsbymovingbackandforthbetweendifferentwordsandexpressions.
Mr.Bakbelievesthiseffortimproveswhatscientistscalledexecutivefunctioningorattentiontotasks,thismentalabilityoftenweakensinpeoplewithdementias.
Researchersfoundtherewasnoextragaininspeakingmorethantwolanguages.TheyalsodidnotseeadelayinthefirstsignsofLewybodiesdementia(路易体痴呆症),thedisordercausespatientstoseeorexperiencethingsthatdonotrealexist.Theycanalsocausesuffererstomovebackandforthbetweenbeingwideawakeandreallysleeping.
Mr.Baksaysitdoesnotappearimportantwhetheryoulearnalanguageatayoungageorlaterinlife.
"Soit'snotsomethingyousortofsaythat'youmissedtheboatwhenyoudonotdoitasababy'.Itissomethingthatisstillquiteusefulandpowerfulwhenyoudoitasanadult,"hesaid.
Scientistsfoundthatspeakingmorethanonelanguagehelpdelaythefirstsignsofdementias,eveninthosewhocouldnotread.
Whichoneisthemostcommonphenomenonofmentaldecline? A．Losingtheabilitytospeak.
B．Failingtorecognizethings.
C．Alzheimer'sdisease.
D．Beingunabletofallasleep.
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