换流阀不开通故障是由（）引起的。
A.丢脉冲
B.门极控制回路故障
C.直流线路故障
D.电流差流
参考答案：A, B
But for the metal cage to pull the miners to safety, the miners in Chile ____ in less than twenty-four hours.
A．would not be rescued
B．would not have been rescued
C．would be rescued
D．would have been rescued
属于直根系的有（）
A.定根
B.不定根
C.主根
D.侧根
E.纤维根
