简述作用在起重机上的外负荷。
参考答案：
作用在起重机上的外载荷有起升负荷、自重载荷、动载荷、风载荷、货物偏摆载荷、碰撞载荷、工艺载荷、安装和运输载荷等。
为什么光纤中只能传输有限个导模？
There are some health problems that, when ______ in time, can become bigger ones later on.
A．not treated
B．not being treated
C．not to be treated
D．not having been treated
