The correct answer is D. gravity. Gravity plays a crucial role in the water cycle, allowing for the occurrence of precipitation and runoff. When water vapor condenses in the atmosphere, it forms clouds. Eventually, the water droplets in clouds become large enough to fall to the ground as precipitation, such as rain, snow, or hail. Precipitation that does not evaporate or infiltrate into the ground then flows across the Earth's surface as runoff, thanks to the force of gravity pulling it downward.
