You're all wrong!
There are 2 case plays that cover similiar situations. In both plays, B actually inbounds the ball. But since neither is an officials mistake, it is correctable. In both, if there is any confusion as to who's ball it, which there appeared to be in this play, then no action is taken against B. If the act were judged to be deliberate, it would be a T for delay of game.
In this case, I would simply pick the ball up and administer the throw-in again.