Answer the following question given this paragraph: 

The sliding-and-shortening process occurs all along many myofibrils and in many muscle fibers. It causes the muscle fibers to shorten and the muscle to contract.


Q: What part of the muscle fibers cause shortening and contraction?

 Choices:

- fasciae

- pores

- collagen

- myofibrils

A:
myofibrils
