A:胚珠
B:花药
C:雌蕊
D:子房
Look at this plant. It is green. It has big leaves.
— Could you tell me _____? I must find him.
— Sorry, I have no idea. But he was here just now.
如图所示,滑轮下挂重400N的物体G,滑轮重20N,绳和杠杆都是轻质的.要在图示位置使杠杆平衡,在杠杆的A点所加的竖起向上的力F应是(杠杆上标度的间距相等)( )