在下述黏膜组织中，何处无黏膜下层（）.
A.颊
B.唇
C.舌腹
D.口底
E.龈
参考答案：E
David suddenly felt nervous. ___________, it was his first time to go on stage.[ ]
A. At least
B. After all
C. At once
D. At all
负责给塔底电控系统供电的断路器是（）.
A.2F10
B.2F12
C.2F6
D.2F4
