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1、1994Passage4 "Ihavegreatconfidencethatbytheendofthedecadewe'llknowinvastdetailhowcancercellsarise,"saysmicrobiologistRobertWeinberg,anexpertoncancer."But,"hecautions,"somepeoplemayhavetheideathatonceoneunderstandsthecauses,thecurewillrapidlyfollow.ConsiderPasteur.Hediscoveredthecausesof
2、manykindsofinfections,butitwasfiftyorsixtyyearsbeforecureswereavailable." Thisyear,50percentofthe910000peoplewhosufferfromcancerwillsurviveatleastfiveyears.Intheyear2000,theNationalCancerInstituteestimates,thatfigurewillbe75percent.Forsomeskincancers,thefive-yearsurvivalrateisashighas90
3、percent.Butothersurvivalstatisticsarestilldiscouraging—13percentforlungcancer,and2percentforcancerofthepancreas. Withasmanyas120varietiesinexistence,discoveringhowcancerworksisnoteasy.Theresearchersmadegreatprogressintheearly1970s,whentheydiscoveredthatoncogenes,whicharecancer-causingge
4、nes,areinactiveinnormalcells.Anythingfromcosmicraystoradiationtodietmayactivateadormantoncogene,buthowremainsunknown.Ifseveraloncogenesaredrivenintoaction,thecell,unabletoturnthemoff,becomescancerous. Theexactmechanismsinvolvedarestillmysterious,butthelikelihoodthatmanycancersareinitiat
5、edatthelevelofgenessuggeststhatwewillneverpreventallcancers."Changesareanormalpartoftheevolutionaryprocess,"saysoncologistWilliamHayward.Environmentalfactorscanneverbetotallyeliminated;asHaywardpointsout,"Wecan'tprepareamedicineagainstcosmicrays." Theprospectsforcure,thoughstilldistant,
6、arebrighter. "First,weneedtounderstandhowthenormalcellcontrolsitself.Second,wehavetodeterminewhethertherearealimitednumberofgenesincellswhicharealwaysresponsibleforatleastpartofthetrouble.Ifwecanunderstandhowcancerworks,wecancounteractitsaction."63.TheexampleofPasteurinthepassageisusedt
7、o________. [A]predictthatthesecretofcancerwillbedisclosedinadecade [B]indicatethattheprospectsforcuringcancerarebright [C]provethatcancerwillbecuredinfiftytosixtyyears [D]warnthatthereisstillalongwaytogobeforecancercanbeconquered64.Theauthorimpliesthatbytheyear2000,__
