飞机结冰是指飞机机体表面某些部位聚集冰层的现象，飞机积冰主要分为三种：（）
A.冰.雾凇.霜
B.明冰.毛冰.白冰
C.坚冰.松冰.霜冰
参考答案：A
I can't forget the time _______ the earthquake happened in Yushu.[ ]
A. when
B. which
C. where
