Let's look at system C.
Now, in system C we basically have a cell. And this cell is surrounded by semipermeable membrane that allows water molecules in and out but does not allow any size molecules to go in or out. Now, in this cell, however, we have a small hole in the membrane and this hole is large enough to allow the molecules in and out. So water is going to travel in and out and so will the molecules. So eventually, according to what entropy tells us, we're going to want to go from system C to system D. That is, we want to travel from this system to this even system in which we have five molecules on this side on the inside and five molecules on the outside. Now let's look at a third system.