—Did you meet the professor at the airport?

—No，he ____ by the time I ____ there．

A．had left；got

B．has left；reached

C．was leaving；got

D．left；had got

如图所示，用一个重为18N的动滑轮来提升物体，若物体匀速上升的过程中竖直向上的拉力F=30N，（不计绳重和摩擦）则动滑轮的机械效率为       。