《球面发散》
参考答案：
指点震源向外传播时，形成一个球面波前，由于几何扩展的原因，波的强度随着距离的增加而衰减，这种现象称为球面发散。
Wang Xiaobo was a quite special writer. His books are well worth ______. [ ]
A. reading
B. being reading
C. seeing
D. being seen
画出电动机正转控制电路图。
