—Hi,Smith, can I borrow the meeting-room upstair this afternoon?

—Sorry．It ____

A．is painted

B．is being painted

C．has painted

D．had been painted

确定夹紧力方向时，应该尽可能使夹紧力方向垂直于（）基准面。

A、主要定位

B、辅助定位；

C、止推定位