新概念3第一课书后几道习题不懂?1.Pumasneverattackahumanbeingexcept（D）corneredAtheyareBbeingCthattheyareDwhentheyare为什么不能选B?2.Theexpertswerenowfullyconvincedthattheanimal(D)apumaAmustbeB

　　新概念3第一课书后几道习题不懂?

　　1.Pumasneverattackahumanbeingexcept（D）cornered

　　AtheyareBbeingCthattheyareDwhentheyare

　　为什么不能选B?

　　2.Theexpertswerenowfullyconvincedthattheanimal(D)apuma

　　AmustbeBshouldhavebeenCcanonlybeDcouldonlyhavebeen

　　为什么不能选A?

　　3.Aprivatecollectorisamanwhocollects(A）.

　　AforhisownbenefitBonhisownCinprivateDunknowntothepublic

　　为什么不选D?