突出煤层在无突出危险区，石门揭煤工作面到突出煤层的最小法向距离不得小于（）。
A、5m
B、10m
C、20m
参考答案：A
— Why do you do volunteer work in the North-West?
— I_______ to improve the lives of the children there through my efforts.
A．was trying
B．have tried
C．am trying
D．tried
何谓高频保护的通道余量？
