题目内容：
根据以下材料，回答题Inthe21stcenturythere'snodoubtthatfrighteningnewinfectiousdiseaseswillappear.Todaynewvirusesarecomingoutofnatureand"discovering"thehumanspecies.Justsince1994,atleast30newviruseshaveappeared.
Virusesaremovingintothehumanspeciesbecausetherearemoreofusallthetime.Froma
virus'pointofview,welooklikeafreelunchthat'sgettingbigger.InnatureviraldiseasestendtObreakoutwhenpopulationsincreaserapidlyandbecomedenselypacked.Thenmanydeathsoccurandthepopulationdrops.Thisisnature'spopulation-controlmechanism.Thereis-noreasontothinkthehumanraceisfreefromthelawsofnature.
Givingtheselawsanextrapushwillbetheriseofmegacities--hugedenselypackedcitiesinlessdevelopednations.AUnitedNationsstudypredictsthatbytheyear2015,therewillbe26ex-tremelybigcitiesontheplanet.Bythen,somemegacitiescouldhave30millionormorepeople.ThatisapproximatelythetotalpopulationofCalifornia.ImagineallthepeopleinCaliforniacrowd-edtogethertightlyintoonevastcity.Thenremovemostdoctorsandmedicalcare,takeawaybasicsanitationandhygiene,andyouhaveabiological"timebomb".Nowmakeeightortensuch"bombs"andplantthemaroundtheworld.
Alsoconsiderthebiologicalweaponstheworldwillbecapableofproducinginthefuture.The20thcenturysawthecreationofgreatandterribleweaponsbasedontheprinciplesofnuclearphysics.The21stcenturywillseegreatandterribleweaponsbasedontheknowledgeofDNAandthegeneticcode.Asbiotechnologybecomesmoresophisticatedandpowerful,biologistswilllearnhowtomixgenesofdifferentmicrObestocreateunnaturalstrainsthatcanbeturnedintodeadly,ef-fectiveweapons.
Biologicalweaponsareadisgracetobiology.Mostbiologistshaven'twantedtotalkoreventhinkaboutthem.Thephysicistslosttheirinnocencewhenthefirstnuclearbombwentoffin1945.Thebiologistswilllosetheirinnocencewhenthefirstbiologicalweaponspreadsthroughthehumanspecies.
Yetthe20thcenturysurviveddespitetheexistenceofthenuclearbomb.Therewasgreateco-nomicandscientificprogressandmuchhumanhappiness.Thesamecanbetrueinthenextcentury.Wemaynotcompletelywinthe21stcenturymicrobewar,butIamconfidentthatwewon'tloseit.
InfectiousdiseasesaremorelikelytoOccurwhere A．differenthumanracesaremixedinliving.
B．manypeopleliveincrowdedcommunities.
C．populationdropsduetonature’scontrolmechanism.
D．newvirusesappearforthefirsttime.
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