he's the one that started the thing by putting his bat somewhere that it could get run over by the person driving the car out of the garage. But to be fair, since he's never backed the car out himself, he probably doesn't really think about that as he's setting the bag down.
Hard to imagine C not feeling the bag being dragged, but clearly C would have stopped if they realized it.
I think you have to chalk it up as an expensive lesson about not leaving stuff right behind the car.
Full disclosure - I've backed out of my garage twice and scraped cars in the driveway that is normally clear. Just in a hurry, and used to throwing it in reverse, checking the mirror but not over the shoulder, and I scraped two different cars.