What happens next?

The animated graphic morphs into a real and actual version of the graphic, appearing on a outdoor tennis court and flanked by two men. the two men standing next to the machinery

pick from the following.
[A]. place tennis balls into the machine, which in turn spits out the balls acting as an automated tennis ball server for a player who is standing on the other side of the tennis court with a tennis racket.
[B]. are introduced and they were referring to the equipment that the man is demonstrating.
[C]. and preparing to demonstrate an exercise called javelin throw.
[D]. are doing various than normal exercises in consecutive activities, interspersed with occasional action in an unsuccessful attempt to aid in improving tennis.
[A].