_______ in the city for thirty years, the old man moved back to the countryside.

A．Having worked

B．Working

C．To have worked

D．To work

在我国，二尖瓣关闭不全的最常见病因是（）

A.二尖瓣脱垂

B.风湿性心脏病

C.感染性心内膜炎

D.二尖瓣环和环下部钙化

E.冠心病