Every student as well as teachers who_______ to visit the museum ______ asked to be at the school gate on time.

A．is; is

B．are; are

C．is; are

D．are; is

第一心音的产生主要是由于（）。

A.半月瓣关闭

B.主动脉瓣关闭

C.肺动脉瓣关闭

D.房室瓣关闭

E.房室瓣开放