Which best explains why we are able to accelerate forward when starting to run? A) The runner's upper body quickly leans forward, causing the entire body to begin accelerating forward. B) As one leg moves backward, it provides an opposite force for the other foot to move forward. C) The foot not touching the ground propels the entire body as it swings forward. D) The striking foot pushes backward against the ground. The friction with the ground provides an equal and opposite force forward.