4. If a bird pushes air downward with its wings, the reaction force is... __ A. The air pushing upward on the bird. B. Gravity pulling the bird downward. C. The ground pushing the bird upward. D. The wind blowing against the bird.

. The air pushing upward on the bird.<br /><br />This is an example of Newton's Third Law of Motion: For every action, there is an equal and opposite reaction. The action is the bird pushing down on the air. The reaction is the air pushing upward on the bird, enabling it to fly.<br />