引导步是用于走在（）的步态。
A.迎接客人
B.前面给客人带路
C.送别客人
D.上下班时
参考答案：B
— Did you see the beginning of the film?
— No. It ________ for 20 minutes when I got to the cinema. [ ]
A. has been on
B. had been on
C. had begun
D. has begun
定心夹紧机构具有定心同时将工件（）的特点。
A.定位
B.校正
C.平行
D.夹紧
