The center of flotation of a vessel is（）.
题型：单项选择题
问题：
The center of flotation of a vessel is（）.
A.the center of volume of the immersed portion of the vessel
B.the center of gravity of the water plane
C.that point at which all the vertical downward forces of weight are considered to be concentrated
D.that point at which all the vertical upward forces of buoyancy are considered to be concentrated