开放进站信号机最理想的时机是列车头部正好运行在（）的地点。
A.进站信号机
B.进站信号机前200m
C.预告信号机
D.预告信号机前200m
参考答案：D
The government has taken some measures to solve the shortage of electricity, but it may be some time ____________ the situation improves.
A．since
B．when
C．unless
D．before
弯曲模试冲时，冲压件弯曲部分产生裂纹的原因是什么？解决方法？
