You are presented with the question "What causes the pull of gravity?" and the following answer choices: 
- layers of fat
 - Earth's mass
 - Time and energy
 - flow of electrons
 - kinetic energy
 - friction
 - an object
 - heat energy

Now knowing that the mass of a planet causes the pull of gravity on that planet and earth is a living planet, choose the best answer.


 

  Earth's mass       