颊侧翼缘区的前缘是（）

A.下颌骨外缘

B.牙槽嵴的颊侧斜坡

C.唇系带

D.颊系带

E.磨牙后垫

It is futile to argue with him once he has made up his mind.

A．unpleasant

B．encouraging

C．helpful

D．useless