I can think of many cases___    _ students obviously knew a lot of English words and expressions but

couldn't write a good essay.    [ ]

A．why    

B．which    

C．as      

D．where

常引起脊柱不稳的脊椎骨折是（）

A.椎体前柱骨折

B.椎体中柱骨折

C.椎体后柱骨折

D.椎体压缩性骨折

E.以上都是