If some backboned aquatic animals have a circulatory system with a two-chambered heart, then the two-chambered heart of some backboned aquatic animals is part of their:?

Answer choices:
- aerobic capacity
 - three-chambered heart
 - biological diversity
 - it keeps an organism warm
 - the body's largest organ
 - circulatory system
 - salmon
 - swimming system

 

      circulatory system    