Answer the following question given this paragraph: 

Organs of the GI tract are covered by muscles that contract to keep food moving along. A series of involuntary muscle contractions moves rapidly along the tract, like a wave travelling through a spring toy. The muscle contractions are called peristalsis. The diagram in Figure below shows how peristalsis works.


Q: What do the muscles of the gastrointestinal tract do?

 Choices:

- break down food

- secrete stomach acid

- stop digestion

- keep food moving

A:
keep food moving
