Precipitation is the condensation of water vapor to droplets. It forms clouds that when it contains a lot of water droplets will fall into the ground, and we call it "rain".
Runoff is when the flow of water is in excess and the land cannot absorb it fast.
In both processes, gravity acts on the flow of water. During precipitation, gravity pulls the droplet down from the atmosphere. During a runoff, the flow of water goes down from the side of the mountains to the streams and rivers. Both can also be observed in a water cycle.
ANSWER: B. gravity