Answer the following question given this paragraph: 

Cardiac muscle is found only in the walls of the heart. When cardiac muscle contracts, the heart beats and pumps blood. Cardiac muscle contains a great many mitochondria, which produce ATP for energy. This helps the heart resist fatigue. Contractions of cardiac muscle are involuntary like those of smooth muscle.


Q: What muscle is found only in the walls of the heart?

 Choices:

- respiratory muscle

- abdominal muscle

- skeletal muscle

- cardiac muscle

A:
cardiac muscle
