A U$-$tube that is open on both ends has a moveable disc in it that can move frictionless like a piston 

from left to right, but that keeps matter from crossing from one side of the tube to the other. Liquid

water $($d$=1\mathrm{.}0$ g$/$cm$3)$ is added to one side of the tube. Liquid ethanol $($d $=0\mathrm{.}798$ g$/$cm$3)$ is added to the

side of the tube. In order to keep the disk from moving, how will the volumes of water and ethanol

compare? a$)$ more water must be added

b$)$ more ethanol must be added

c$)$ equal amounts of water and ethanol must be added