Answer the following question given this paragraph: 

Vertebrates have a system of muscles attached to the endoskeleton to enable movement. Muscles control movement by alternately contracting (shortening) and relaxing (lengthening). Generally, muscles work together in opposing pairs.


Q: In vertebrates, a system of what structures attached to the endoskeleton enables movement?

 Choices:

- nerves

- tendons

- tissues

- muscles

A:
muscles
