题目内容：
Questionsarebasedonthefollowingpassage:
IntheUnitedStates,30percentoftheadultpopulationhasa"weightproblem".Tomanypeople,thecauseisobvious:theyeattoomuch.Butscientificevidencedoeslittletosupportthisidea.GoingbacktotheAmericaofthe1910s,wefindthatpeoplewerethinnerthantoday,yettheyatemorefood.Inthosedayspeopleworkedharderphysically,walkedmore,usedmachinesmuchlessanddidn'twatchtelevision.
Severalmodernstudies,moreover,haveshownthatfatterpeopledonoteatmoreontheaveragethanthinnerpeople.Infact,someinvestigations,suchasthe1979studyof3,545Londonofficeworkers,reportthat,onbalance,fatterpeopleeatlessthanslimmerpeople.
Studiesshowthatslimpeoplearemoreactivethanfatpeople.AstudybyaresearchgroupatStanfordUniversitySchoolofMedicinefoundthefollowinginterestingfacts:
Themorethemenran,themorebodyfattheylost.
Themoretheyran,thegreateramountoffoodtheyate.
Thus,thosewhoranthemostatethemost,yetlostthegreatestamountofbodyfat.
ThephysicalproblemthatmanyadultAmericanshaveisthat（ ）. A．theyaretooslim
B．theyworktoohard
C．theyaretoofat
D．theylosetoomuchbodyfat
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