[KWH]

I would treat it the same as an inadvertant whistle.
Once the official took possession of the LIVE BALL the ball became dead.
Since B was last team in possession, I would give B the option of replaying the down or taking possession of the ball at the spot where the official killed the play by taking possesion of a live ball.

This decision is supported by NFHS Rule 1-6-1

Editors note: Most people would not like their children to be within earshot of my post game discussion with that official.