Problem Solving Help!?

Just four boys start in the 3:15 race, a short sprint. They are Adam, Ben, Chris and David. At the first turn Ben is ahead of David and Chris is ahead of Adam, but nothing more specific is known. By the half way mark the only change is that David has dropped back one place. Then, in the last furdog, Ben moves up one place and that is how they finish. On the basis of this information, who came 1st, 2nd and 3rd?