风湿性心内膜炎最常受累的瓣膜为( )
A．主动脉瓣
B．二尖瓣
C．三尖瓣
D．二尖瓣及三尖瓣
E．肺动脉瓣
参考答案：B
When you got there, how many guests in the meeting hall?
A．had been
B．have been
C．were
D．have had
杜仲的（）是药材。
A、树根
B、树叶
C、树枝
D、树皮
