反锁牙合是指（）

A.上后牙的近中颊尖位于下后牙的中央窝，牙合面有咬合接触

B.下后牙的远中颊尖位于上后牙的中央窝，牙合面有咬合接触

C.上后牙的舌斜面位于下后牙颊斜面的颊侧，牙合面无咬合接触

D.上后牙颊尖的颊斜面位于下后牙舌尖舌斜面的舌侧，牙合面无咬合接触

E.上后牙的舌斜面位于下后牙颊斜面的舌侧，牙合面有咬合接触

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