Nothing to be sorry about. This a great way to learn. I made the same mistake that you did when I first read this play. That is why I posted it.

Loose ball personal foul face-mask. The basic spot for enforcement of a loose-ball foul is the preceeding spot. A can accept the penalty which would put the ball on B's 45, and replay the down, or they can decline the penalty which would put the ball on B's 45 but, third down