Answer:
Action and reaction forces have the same magnitude but act in opposite directions
Explanation:
When the swimmer pushes against the water (an action force), the water pushes back on the swimmer. This reaction force is what propels the swimmer in the forward direction.
Now the magnitude of the action and the reaction force is exactly the same. This means that the harder the swimmer pushes onto the water, the harder it pushes back.
However, the direction of the action force is opposite the direction of the reaction force. The swimmer pushes onto the water in the backward direction, the water pushes onto the swimmer in the forward direction.