因为线段a垂直于线段b，线段b垂直于线段c，所以线段a垂直于线段c。[ ]
错误
He works very hard. Every night he doesn’t go to bed _____eleven o’clock. [ ]
A. after
B. since
C. in
D. until
椎体压缩部分占椎体高度的1／2属于：（）
A.Ⅰ°
B.Ⅱ°
C.Ⅲ°
D.Ⅳ°
E.Ⅴ°
