three times，but she still couldn’t follow what the old man said．

A．Having told

B．Having been told

C．She had been told

D．Though he had been told

期前收缩的发生原理是（）

A.窦性心律失常

B.被动性异位心律失常

C.主动性异位心律失常

D.生理性冲动传导异常

E.病理性冲动传导异常