Choose from:
[a]. layers of fat
[b]. Earth's mass
[c]. Time and energy
[d]. flow of electrons
[e]. kinetic energy
[f]. friction
[g]. an object
[h]. heat energy
Given those answer options, answer the question: 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, what causes the pull of gravity?
A:
[b].