Joe, my close friend, moved to Beijing years ago, and I             him since then. [ ]

A. don't see

B. didn't see

C. haven't seen

D. won't see

在室内高压设备上工作，应在工作地点两旁间隔和对面间隔的遮栏上和禁止通行的过道上悬挂什么标示牌？