求解新GRE《阅读逻辑10套》EX4第五题的D选项!Peoplewhoengageinscubadivingarehealthier,onaverage,thanpeoplewhodonotengageinthisactivity.Therefore,scubadivingtendstopromoteimprovedhealth.Theargumentismostvulnerabl

　　求解新GRE《阅读逻辑10套》EX4第五题的D选项!

　　Peoplewhoengageinscubadivingarehealthier,onaverage,thanpeoplewhodonotengageinthisactivity.Therefore,scubadivingtendstopromoteimprovedhealth.

　　Theargumentismostvulnerabletocriticismonthegroundsthatit

　　（A）presupposesthateveryonewhotakesupscubadivingdoessosolelyforhealthreasons

　　（B）leadstoafurtherandfalsifiableconclusionthatnoonecanachievegoodhealthwithoutengaginginscubadiving

　　（C）failstopointoutthatasmallnumberofpeopleareseriouslyinjuredinscubadivingaccidentseachyear

　　（D）treatsapreconditionforimprovingone‘shealthasthoughitweresomethingthatbyitselfcouldensuregoodhealth

　　（E）overlooksthepossibilitythatpeoplegenerallydonottakeupscubadivingunlesstheyareingoodhealth

　　为什么不选D?选项里的precondition是指什么啊?