题目

It is rather difficult to make friends with her, but her friendship, _______, is truer than any other’s. A．while gaining          B．after gaining          C．when to gain           D．once gained 答案：D蜜蜂和蝴蝶同时从你脑后经过，为什么能感觉到蜜蜂的存在而感觉不到蝴蝶的存在？