She was amazed by               in the mainland at the age of 6. [ ]

A. what she saw

B. what did she see

C. where she saw

D. where did she see

关于龈下牙石，叙述正确的有（）

A.龈下牙石位于龈缘下方

B.与龈上牙石相比，牙面附着更紧密

C.牙石常呈褐色或黑色

D.牙石多位于牙齿的邻面

E.X线片上不能观察到