题目内容：
根据以下材料，回答题Whendidyoulastseeapolarbear?Onatriptoazoo,perhaps?IfyouhadattendedawinteractivityinNewYorkafewyearsago,youwouldhaveseenawholepolarbearclub.These"PolarBears"
arepeoplewhomeetfrequentlyinwintertoswiminfreezingcoldwater.Thatday,theairmtemperaturewas3C,andthewatertemperaturewasabithigher.ThemembersofthePolarBear
Clubat
ConeyIsland,NewYorkareusuallyabouttheageof60.Membersmustsatisfytworequire-mments.First,theymustgetalongwellwitheveryoneelseinthegroup.Thisisveryimportant
becausemthereare
somanydifferentkindsofpeopleintheclub.PolarBearsmustalsoagreetoswimoutdoorsatleasttwiceamonthfromNovemberthroughFebruary.
Doctorsdonotagreeaboutthemedicaleffectsofcold-waterswimming.Someareworriedaboutmthedangersofaconditioninwhichthebody'stemperaturedropssolowthatfinallytheheartstops.
Otherdoctors,however,pointoutthatthereismoredangerofaheartattackduringsummerswimmingbecausethedifferencebetweentheairternperatureandwatertemperatureismuchgreater
insummerthaninwinter.
ThePolarBearsthemselvesaresatisfiedwiththebenefits(oradvantages)ofcold-waterswim-mruing.Theysaythattheirfavoriteformofexerciseisverygoodforthecirculatorysystembecauseitforcesthebloodtomovefasttokeepthebodywarm.Cold-waterswimmersusuallyturnbrightredafterafewminutesinthewater.Apersonwhoturinblueprobablyhasaverypoorcircula-torysystemandshouldnottrycold-waterswimming.
Themainbenefits(oradvantages)ofcold-waterswimmingareprobablymental.ThePolarBearsmlovetoswimyear-round;theyfinditfunandrelaxing.Asone70-year-oldwomansays,"WhenIgointothewater,Ipourmytroublesintotheoceanandletthemfloataway."
Doctors A．encouragepeopletotakepartincold-waterswimmingactively
B．pointoutthepossibledangerofbloodillnessduringcold-waterswimming
C．beheveswimmingishelpfulbothinwinterandinsummer
D．havedifferentideasaboutthemedicaleffectsofcold-waterswimming
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答案解析：