You are presented with the question "What causes water to stay on earth's surface?" and the following answer choices: 
- Clouds
 - Planets
 - rainfall
 - friction
 - Gravity
 - sun's heat
 - Dirt
 - forces

Now knowing that gravity pulls objects towards planets and because of gravitation, objects that are on or near the earth are pulled toward it, choose the best answer.


 

     Gravity    