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

Fact 2: Mass, the amount of matter in an object, is measured in grams.

Given the two facts above, answer the question "what part of a planet causes the pull of gravity on that planet?" with the following options: 
- solid, liquid, gas
 - layers of fat
 - barrel-shaped
 - our star
 - amount of matter
 - an external framework
 - hydrogen and oxygen
 - It expands


 

     amount of matter    