题目内容：
根据以下材料，回答题 ScreenTest
1．EveryyearmillionsofwomenarescreenedwithX-raystopickupsignsofbreastcancer．Ifthishappensearlyenough,thediseasecanoftenbetreatedsuccessfully．Accordingtoasurveypublishedlastyear,21countrieshavescreeningprogrammes．Nineofthem,includingAustralia,Canada,theUSandSpain,screenwomenunder50.
2．Butthemedicalbenefitofscreeningtheseyoungerwomeniscontroversial,partlybecausetheradiationbringsasmallriskofinducingcancer．Also,youngerwomenmustbegivenhigherdosesofX-raysbecausetheirbreasttissueisdenser.
3．ResearchersatthePolytechnicUniversityofValenciaanalysedtheeffectofscreeningmorethan160,000womenat11localclinics．Afterestimatingthewomen'sincreasingdoseofradiation,theyusedtwomodelstocalculatethenumberofextracancersthiswouldcause.
4．ThemathematicalmodelrecommendedbyBritain'sNationalRadiologicalProtectionBoard(NRPB)predictedthatthescreeningprogrammewouldcause36cancersper100,000women,18ofthemfatal．ThemodelpreferredbytheUNScientificCommitteeontheEffectsofAtomicRadiationledtoalowerfigureof20cancers.
5．Theresearchersarguethatthelevelofradiation-inducedcancersis"notverysignificant"comparedtothefarlargernumberofcancersthatarediscoveredandtreated．TheValenciaprogramme,theysay,detectsbetween300and450casesofbreastcancerinevery100,000womenscreened.
6．Buttheypointoutthattheriskofwomencontractingcancerfromradiationcouldbereducedbybetween40and80percentifscreeningbeganat50insteadof45,becausetheywouldbeexposedtolessradiation．Theresultsoftheirstudy,theysuggest,couldhelp"optimise(完善)thetechnique"forbreastcancerscreening.
7．"Thereisatrade-off(平衡)betweenthediagnosticbenefitsofbreastscreeninganditsrisks,"admitsMichaelClarkoftheNRPB．Buthewarnsthatthestudyshouldbeinterpretedwithcaution．"Onthebasisofthecurrentdata,forevery10cancerssuccessfullydetectedandpreventedthereisariskofcausingonelaterinlife．That'swhyradiationexposureshouldbeminimisedinanyscreeningprogramme."
Paragraph2__________. A．Harmscreeningmaydotoayoungerwoman
B．Smallriskofinducingcancersfromradiationmodels
C．Investigatingtheeffectofscreening
D．Effectspredictedbytwoaredifferent
E．Treatmentofcancers
F．Factorsthattriggercancers
参考答案：
答案解析：