An object in motion eventually stops. The ball must have been going slightly uphill. The ball did not have enough mass to keep going. There must have been an outside force acting on the ball.
I gave my students this multiple choice question: A ball rolling on the ground is observed to slow down and stop. Based on Newton's first law of motion, what is the best conclusion?

Only one answer is correct among these 4 choices:
- An object in motion eventually stops.
- The ball must have been going slightly uphill.
- The ball did not have enough mass to keep going.
- There must have been an outside force acting on the ball.

Could you tell me which one is correct?

There must have been an outside force acting on the ball.