Final answer:
The runner's striking foot pushes backward against the ground, as explained by Newton's 3rd law of motion, allowing the runner to accelerate and run.
Step-by-step explanation:
The correct answer is: The striking foot pushes backward against the ground. When a runner is able to accelerate and run, Newton's 3rd law of motion comes into play. According to this law, for every action, there is an equal and opposite reaction. When the runner's striking foot pushes backward against the ground, the ground exerts an equal and opposite force forward on the foot. This force propels the runner forward and allows them to accelerate and run.
For example, when you push against a wall, the wall pushes back with an equal amount of force. Similarly, when a runner pushes against the ground, the ground pushes back with an equal force, propelling the runner forward.
Learn more about Newton's 3rd law of motion