题目内容：
根据以下材料，回答题
MentalRetardation(智力迟钝)
Mentalretardationisaconditioninwhichpeoplehavelowerintelligencethannormalandareunabletofunctionatthelevelexpectedfortheirage．Peoplewithmentalretardationareusuallybornwithit,oritdevelopsearlyintheirlife．Theymayalsohavesomedifficultywithdailylivingskillssuchaslearningtoreadandwriteandcaringforthemselves.
Doctorsandotherprofessionalsdeterminethatapersonhasmentalretardationbasedontheirintelligenceandhowwelltheycandoeverydayactivities．Intelligenceistheabilitytolearnandunderstand．Levelsofintelligencearemeasuredbyspecialtestscalledintelligencetests．Thescoreapersongetsononeofthesetestsgivesanumerical(数字的)measureofaperson'sintelligence.
Thisiscalledanintelligencequotient(智商)orIQ.
AnaveragescoreonanIQtestisabout90to110．Apersonwithmentalretardationwillusuallyscorebelow75onanIQtest.
TheIQtestalonedoesnotdeterminewhethersomeoneismentallyretarded．Apersonmustalsohavetroublewitheverydayactivitiessuchasgettingdressed,eating,washing,orlearningbasicreadingandwritingandarithmeticskills.
Mentalretardationisnotadiseaseitself．Itcannotbecuredandit'snotcontagious(传染的).
Thisconditioncanbecausedbyseveralthingsthatinjurethebrainordon'tallowthebraintodevelopnormally．Manytimeswedon'tknowwhyapersonhasmentalretardation.
Sometimesitmaybecausedbygenes．Genesarechemicalunitsfoundineverycell．Theycarrytheinstructionstellingcellswhattodo．Sometimes,childrenreceiveabnormalgenesfromtheirparents.
Adefectivegenemayalsodevelopspontaneously．Neitherparentwouldhavepassedonthegene,butthegenechangesbeforethebabydevelops.
Someotherproblemsthatcancausementalretardationalsohappenbeforeachildisborn．Itisimportantforthebraintodevelopproperlyifachildistohavenormalintelligence．"Planningapregnancyisthemostimportantdecisionmostofuswillmakeinourlifetime．Ahealthylifestyleandgoodmedicalcareshouldbeginbeforeawomanbecomespregnantandcontinuethroughoutthepregnancy,"saysJodiRucquoi,ageneticcounselorfromConnecticut．Ifawomanabusesalcoholordrugsordoesn'teatwell,thereisarisktothedevelopingbaby．Also,aprematurebirthorproblemsduringchildbirthcansometimesharmthebaby'sdevelopingbrain．Whileprematurebabiesaregenerallyfine,thereisagreaterchancethattheymayhavementalretardation.
Insomecases,ayoungchildcandevelopmentalretardationafterbeingsickwithaseriousinfectionorotherillnesses,oraftersufferingabadheadinjury.
Achildcanbetakenasmentallyretardedifheorshe__________. A．wasbornprematurely
B．haslowerintelligencethannormal
C．haslowintelligenceandlowdailylivingskills
D．hasnormalintelligencebutdifficultiesinlearning
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