Why does your heart never tire of constantly beating to pump blood throughout your body?

A.
It belongs to the skeletal muscle group, which means it moves involuntarily.
B.
It consists of fibers that move voluntarily, so you can train these fibers to increase their endurance level.
C.
It has a high level of muscular strength due to extensive weight training.
D.
It consists of cardiac muscle tissues, which means it moves involuntarily.