酸蚀无法起到的作用有（）。
A.增大牙齿表面积
B.机械清洁作用
C.保护牙髓活力
D.暴露清洁新鲜的釉质
E.A+B+D
参考答案：C
In my dream, I in an ocean of paper.[ ]
A. swims
B. always swimming
C. am swim
D. was swimming
