This sounds like a trick question - it depends on perspective. If you turn the bottle upside down to drink from it then the opening is at the bottom and so is the cola. On the other hand, if we agree that the opening is always the top of the bottle regardless of the bottles orientation, then clearly the cola you're drinking is from the top of the bottle; that is the only path by which the cola can escape the bottle. Meanwhile cola from the bottom of the bottle replaces the cola lost down your throat.
Bigguy; Rochester, New York, U.S.A.