Answer the following question given this paragraph: 

Planets are held in their orbits by the force of gravity. What would happen without gravity? Imagine that you are swinging a ball on a string in a circular motion. Now let go of the string. The ball will fly away from you in a straight line. It was the string pulling on the ball that kept the ball moving in a circle. The motion of a planet is very similar to the ball on a string. The force pulling the planet is the pull of gravity between the planet and the Sun.


Q: What force holds planets in their orbits?

 Choices:

- magnetism

- centrifuge

- Big Bang

- gravity

A:
gravity
