如图所示的装置，滑轮自重20N，其下端挂重280N的物体G，杠杆可绕水平轴无摩擦地转动，绳和杠杆都是轻质的，杠杆上标度的间距相等。要在如图所示水平位置平衡，在杠杆的A点所加的竖直向上的力F应是(      )

A．150N

B．200N

C．300N

D．400N

Which of the following types of profit is most likely the level of accounting profit needed to just cover the implicit opportunity costs ignored in accounting costs（）

A. Normal profit.

B. Economic profit.

C. Accounting profit.