题目内容：
Questionsarebasedonthefollowingpassage:
Weallknowthatitispossibleforordinarypeopletomaketheirhomesontheequator(赤道),althoughoftentheymayfeeluncomfortablyhotthere．Millionsdoit．ButasfortheNorthPole(北极)----weknowthatitisnotonlyadangerouslycoldplace,butthatpeoplelikeyouandIwouldfinditquiteimpossibletolivethere．Atthepresenttimeonlythescientistsandexplorerscandoso,andtheyusespecialequipment．Menhavebeentravellingacrossandaroundtheequatoronwheels,ontheirfeetorinshipsforthousandsofyears;butonlyafewmen,withgreatdifficultyandinveryrecenttime,haveevercrossedtheicetotheNorthPole．Soitmaysurpriseyoutolearnthat,whentravelingbyair,itisreallysafertoflyovertheNorthPolethanovertheequator．Ofcourse,itisnottrueaboutlandingsinthepolarregion,buttheweather,ifweareflyingataheightof5,000metersabovethePoleisadelight．At4,000metersandmoveabovetheearthyoucanalwaysbesurethatyouwillnotseeacloudintheskyasfarasyoureyescanreach．Inthetropics(热带),ontheotherhand,youarenotcertaintokeepclearofbadweatherevenatsuchheightsas18,000or20,000meters．
Aeroplanescan'tclimbashighorasquicklyincoldairasinwarm．Norcanclouds．Inpractice,thisisanadvantagetotheaeroplane,whichisalreadyatagoodheightwhenitreachesthepolarregionandsodoesnottoclimb,whileatthesametimecoldairkeepsthecloudsdownlow．
"Millionsdoit．"inthissentence"doit"refersto（ ）． A．feelinguncomfortablyhotontheequator
B．flyingovertheNorthPole
C．flyingovertheequator
D．makinghomesontheequator
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答案解析：