声是由物体的_ _产生的，声的传播需要__。
振动 介质
物体的振动产生声音；声音的传播需要介质．
故答案为：振动；介质
28. You have done wrong to Jack, _____you should say sorry to him.
A．to which
B．for which
C．in which
D．on which
市话电缆维护的要求是什么？
