The ball is too high up for B to knock down - either it needs to be one block lower (and sitting on a cylinder like it is), or the metal block under it needs to be movable. A good level otherwise, a neat design, and easy enough to get through with careful timing. One other thing that should be noted is the fact that exit blocks are two-sided - B can actually trigger the exit from underneath the exit block if you maneuver him just right in the tunnel.
@D.S.Masters The ball was deliberately placed too high so that it is impossible to reach, so that there is only one way to solve it. It is through the win block underneath.
