为了比较物体运动的______，我们引入了速度这个物理量．一个人步行速度约为1.2m/s，表示的物理意义是

______．

For a floating vessel, the center of buoyancy and the metacenter are in the line of action of the buoyant force（）.

A.only when there is positive stability

B.only when there is negative stability

C.only when there is neutral stability

D.at all times