最常见的联合瓣膜病是

A．二尖瓣狭窄合并二尖瓣关闭不全

B．二尖瓣狭窄合并主动脉瓣关闭不全

C．二尖瓣关闭不全合并主动脉瓣关闭不全

D．二尖瓣关闭不全合并主动脉狭窄

E．主动脉瓣狭窄合并三尖瓣关闭不全

—How long has he been studying English?

—         he was six years old.[ ]

A.Since  

B.For    

C.At    

D.On