–Where are the students? How could they keep us waiting for so long?

–They started an hour ago and they ____ be here in fifteen minutes, I think.

A．can

B．would

C．should

D．must

急性龋的临床表现为（）。

A.多见于中、老年人

B.病变进展较平稳

C.龋洞内腐质湿软

D.龋坏牙本质着色深

E.进展与全身情况无关