Another Play:

[IAUMP]

It seems to me that while B did take the ball into the endzone, they fumbled the ball out of the endzone. The ball was bouncing so it was a grounded loose ball at the time it was kicked. The kick applied a new force which then caused to ball to re-enter the endzone and over the endline. The result of the play is a touch back due to A applying a force to the ball, causing it to go over the endline. Assuming that the kick was intentional, then we also have a foul on A for an illegal kick. The choices given to B as to take the penalty, and have 1/10 at their 7, or decline the penalty and take the result of the play which is a touchback. Obviously they would decline the penalty and take the ball at the 20.