【Ifyouthinkahigh-factorsunscreen（防晒霜）keepsyousafefromharmfulrays，youmaybewrong．Researchinthisweek'sNatureshowsthatwhilefactor50reducesthenumberofmelanomas（黒瘤）anddelaystheiroccurrence，itcan'tpr】

　　Ifyouthinkahigh-factorsunscreen （防晒霜） keepsyousafefromharmfulrays，youmaybewrong．Researchinthisweek'sNatureshowsthatwhilefactor50reducesthenumberofmelanomas （黒瘤） anddelaystheiroccurrence，itcan'tpreventthem．Melanomasarethemostaggressiveskincancers．Youhaveahigherriskifyouhaveredorblondhair，fairskin，blueorgreeneyes，orsunburneasily，orifacloserelativehashadone．Melanomasaremorecommonifyouhaveperiodicintenseexposuretothesun．Otherskincancersareincreasinglylikelywithlong-termexposure．

　　Thereiscontinuingdebateastohoweffectivesunscreenisinreducingmelanomas---theevidenceisweakerthanitisforpreventingothertypesofskincancer．A2011Australianstudyof1，621peoplefoundthatpeoplerandomlyselectedtoapplysunscreendailyhadhalftherateofmelanomasofpeoplewhousedcreamasneeded．Asecondstudy，comparing1，167peoplewithmelanomasto1，101whodidn'thavethecancer，foundthatusingsunscreenroutinely，alongsideotherprotectionsuchashats，longsleevesorstayingintheshade，didgivesomeprotection．Thisstudysaidotherformsofsunprotection---notsunscreen---seemedmostbeneficial．Thestudyreliedonpeoplerememberingwhattheyhaddoneovereachdecadeoftheirlives，soit'snotentirelyreliable．Butitseemsreasonabletothinksunscreengivespeopleafalsesenseofsecurityinthesun．

　　Manypeoplealsodon'tusesunscreenproperly---applyinginsufficientamounts，failingtoreapplyafteracoupleofhoursandstayinginthesuntoolong．Itissunburnthatismostworrying---recentresearchshowsfiveepisodesofsunburnintheteenageyearsincreasestheriskofallskincancers．

　　Thegoodnewsisthatacombinationofsunscreenandcoveringupcanreducemelanomarates，asshownbyAustralianfiguresfromtheirslip-slop-slapcampaign．Soifthereisaheatwavethissummer，itwouldbebestforus，too，tosliponashirt，slopon （抹上） sunscreenandslaponahat．

　　32．WhatdoestheresearchinNaturesayaboutahigh-factorsunscreen___

　　A．Itisineffectiveinpreventingmelanomas．

　　B．Itisineffectiveincaseofintensesunlight．

　　C．Itisineffectivewithlong-termexposure．

　　D．Itisineffectiveforpeoplewithfairskin