The force that opposes the motion of a ball rolling across the ground is friction.

Here's why:

* Rolling Friction: When a ball rolls, there's a small amount of deformation at the point of contact between the ball and the ground. This deformation creates a resistance to the ball's motion, called rolling friction.

* Air Resistance: As the ball moves, it pushes against the air, creating a force called air resistance that opposes the ball's motion. This force is more significant at higher speeds.

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