Fact 1: The mass of a planet causes the pull of gravity on that planet.

Fact 2: Earth is a living planet.

Given the two facts above, answer the question "What causes the pull of gravity?" with the following options: 
- layers of fat
 - Earth's mass
 - Time and energy
 - flow of electrons
 - kinetic energy
 - friction
 - an object
 - heat energy


 

  Earth's mass       