当系统存在二级故障时，风机将（）。
A、正常停机
B、快速停机
C、紧急停机
参考答案：B
—Did you hear about the terrible accident ?
—Yes, but I don’t know _________.
A．where was it happened
B．where it was happened
C．where did it happen
D．where it happened
超滤有何作用？
