A sphere falls through a liquid and eventually reaches terminal speed. Which graph shows the variation with time of the distance travelled by the sphere?

The diagram shows two identical containers, $X$ and $Y$, that are connected by a thin tube of negligible volume. Initially container $X$ is filled with water of mass $m$ up to a height $h$ and $Y$ is empty.

The valve is then opened and both containers contain equal quantities of water. The loss of gravitational potential energy of the water is: