【Animportantfactorthatplaysapartinsusceptibility（易受影响）tocoldsisage．AstudydonebytheUniversityofMichiganSchoolofPublicHealthindicatedthefactthatseemstoholdtrueforthegeneralpopulation．Babiesar】
Animportantfactorthatplaysapartinsusceptibility（易受影响）tocoldsisage．AstudydonebytheUniversityofMichiganSchoolofPublicHealthindicatedthefactthatseemstoholdtrueforthegeneralpopulation．Babiesarethegroupsufferingmostfromcoldsaveragingmorethansixcoldsintheirfirstyears．Boyshavemorecoldsthangirlsuptoagethree．Aftertheageofthreegirlsaremoresusceptiblethanboysandteenagegirlsaveragethreecoldsayeartoboy'stwo．
Thegeneralfrequencyofcoldscontinuestodeclineintoadults．Elderlypeoplewhoareingoodhealthhaveasfewasoneortwocoldsannually．Oneexceptionisfoundamongpeopleintheirtwentiesespeciallywomenwhoshowariseincoldinfections（感染）becausepeopletothisagegrouparemostlikelytohaveyoungchildren．Adultswhodelayhavingchildrenuntiltheirthirtiesandfortiesexperiencethesamesuddenincreaseincoldinfections．
Thestudyalsofoundthateconomics（经济状况）playsanimportantrole．Asincomeincreasesthefrequencyatwhichcoldsarereportedinthefamilydecreases．Familieswiththelowestincomesufferaboutathirdmorecoldsthanfamiliesattheupperend．Lowerincomegenerallyforcespeopletoliveincloserquartersthanthosetypicallyoccupiedbywealthierpeopleandcrowdingincreasestheopportunitiesforthecoldvirustotravelfrompersontoperson．Lowincomemayalsoinfluencediet．Thedegreetowhichpoornutritionaffectssusceptibilitytocoldsisnotyetclearlyprovedbutaninadequatedietissuspectedofloweringresistancegenerally．
68.Whatfactorscausepeopletocatchcoldseasily?
A.Ageandpregnancy． B.Healthandpregnancy．
C.Healthandeconomics． D.Ageandeconomics．
69.Whydoesthelowestincomeleadtotherapidspreadofcolds?
A.Becausepoorpeoplecan'taffordtogotoseeadoctor．
B.Becausebalanceddiethasdefinitelyaffectedpoorpeople．
C.Becausepeoplewiththelowestincomehavelimitedlivingspace．
D.Becausecoldvirusdon'thaveopportunitiestotravelinpoorfamilies．
70.Howdoesthestudyprovethatageplaysapartinsusceptibilitytocolds?
A.Usingcomparison． B.Usinggraphs．
C.Usingdescription．