新概念英语中的几个问题~

新概念英语中的几个问题~第三册中有这个题目：The experts were now fully convinced that the animal ____ a puma A.must be B.should have seen C.can only be D.could only have been.我选择是是A 但答案是D为什么啊还有个.The woman saw"a large cat "___five yards away from her.A.at least B.four or .C,no more than D.within为什么不选择D啊,C为什么有可以.I——it all night.but I'm afraid ...A.shall mend B.am mending C.have mended D.mended 为什么不选择AWe will get used to ___ that .BILLA,hearing B.hear C.the sound D.having heard