— I can’t get through to the general manager’s office.

—The line is busy. Someone ________ the telephone.

A．must have used

B．could use

C．could have used

D．must be using

下列关于空肠的正确叙述是（）。

A.全长包有系膜

B.环状皱襞密而高

C.十二指肠悬肌是确认其起点的标志

D.粘膜上无孤立淋巴滤泡

E.粘膜上无集合淋巴滤泡