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

Fact 2: Gravity accelerates mass.

Given the two facts above, answer the question "What causes a planet to accelerate?" with the following options: 
- other planets
 - magnets
 - wind
 - gravitational pull
 - flow of electrons
 - It gets heated up
 - heat energy
 - energy of moving objects


 

    gravitational pull     