What is the answer?

Which force causes rocks to roll downhill?

OPTIONS:
- gravity
- friction
- erosion
- magnetism
gravity
Vad är svaret? Vilken kraft får stenar att rulla utför? ALTERNATIV: - gravitation - friktion - erosion - magnetism
What's the answer? What force causes stones to roll down? ALTERNATIVE: - gravity - friction - erosion - magnetism
gravitation
gravity