S点为 （）
A.硬腭后部骨棘之尖
B.上齿槽座点
C.上中切牙切缘之最前点
D.下颌角的后下点
E.蝶鞍影像的中心
参考答案：E
We were swimming in the lake__________suddenly the storm started.
A．when
B．while
C．until
D．before
