阵发性室上性心动过速的发生机制主要是

A．心肌缺血

B．折返机制

C．高血压病

D．感染性心内膜炎

E．洋地黄中毒

.The Forbidden City is an area ＿＿ once only the emperor, his family and officials were allowed.

A．which

B．who

C．where

D．whose