Answer the following question given this paragraph: 

The heart is a muscular organ in the chest that consists mainly of cardiac muscle. It pumps blood through blood vessels by repeated, rhythmic contractions. Blood flows through four chambers of the heart when it beats. Heartbeat is controlled by electrical signals from special cells within the heart called the pacemaker.


Q: What type of muscle makes up most of the heart?

 Choices:

- chest muscles

- heart muscle

- respiratory muscle

- cardiac muscle

A:
cardiac muscle
