—What would you like to eat?

—       , please.

A．two piece of breads

B．two breads

C．two pieces of breads

D．two pieces of bread

如图所示，滑轮组的机械效率为80%。在自由端拉力F作用下，重1600N的物体以0.2m/s的速度匀速上升，则10s内对物体做的有用功为______J，拉力的大小为______N，拉力做功的功率为          W。