题目内容：
根据以下材料，回答题 Exercise
Whetherornotexerciseadds（51）thelengthoflife,itiscommonexperiencethatacertain（52）ofregularexerciseimprovesthehealthandcontributestoafeelingofwell-being.
Furthermore,exercise（53）involvesplayandrecreation,andrelievesnervoustensionandmentalfatigue,isnotonlypleasantbutbeneficial.
Howmuchandwhatkindofexerciseoneshould（54）meritcarefulconsideration．Thegrowingchildandthenormalyoungmanandyoungwomanthrilltotheexhilarationofstrenuoussports．Theyfatiguetothe（55）ofexhaustionbutrecoverpromptlywithaperiodofrest．Butnotsowith（56）ofmiddleageandbeyond．Forthemmoderationis（57）vitalimportance.
Justhowmuchexerciseapersonofagivenagecansafelytakeisaquestion（58）toanswer.
Individualvariabilityis（59）greattopermitofgeneralization．Agameoftennismaybeperfectlysafeforonepersonoffortybutfollyforanother．Thesafelimitforexercise（60）ontheconditionoftheheart,theconditionofthemuscles,thetypeofexercise,andtheregularitywithwhichitistaken．Twogeneralsuggestions,however,will（61）assoundadviceforanyone．Thefirstisthattheconditionoftheheartandgeneralhealthshouldbe（62）periodicallybycareful,thoroughphysicalexaminations．The（63）isthatexerciseshouldbekeptbelowthepointofphysical
exhaustion.
Whattypeofexerciseoneshould（64）dependsuponone'sphysicalcondition．Youngpeoplecansafelyenjoyvigorouscompetitivesports,butmostolderpersonsdobettertolimitthemselvestolessstrenuousactivities．Walking,swimming,skatingareamongthesportsthatonecanenjoyandsafelyparticipate（65）throughoutlife．Regularityisimportantifoneistogetthemostenjoymentandbenefitoutofexercise.
回答(51)题 A．for
B．to
C．at
D．of
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答案解析：