Are you going to the concert ______ on Friday evening in the people's Square?   [ ]

A. to hold        

B. to be holding      

C. to have held      

D. to be held

机械设备的防护装置，是通过设置（），将人与危险源隔离的专门装置。

A.警戒标志

B.物体障碍

C.警示标语

D.红色标记