Does it seem reasonable to you that the radius of a circle remains constant?
As the circle rolls there is always a distance equal to the radius between the edge of the circle and the center. So what is the position of the center of the circle as it rolls on a horizontal surface?
Hint: The answer is in the statement above. If you aren't seeing it, you are thinking about the problem too hard. If it helps, take a can and roll it on a table and see what the center of the can does.
-Dan