题目

When he looked up, he suddenly found himself        by a group of teenagers,        looked at him anxiously. A．to be surrounded; which                              B．surrounded; who C．be surrounded; who                     D．having  been surrounded; which 答案：B下图甲为验钞机，乙为超声波测速仪，丙为飞机机翼，它们应用的原理分别是：甲是根据    制成的，乙是根据    制成的，丙是根据    制成的．