Fact 1: Fish have a circulatory system with a two-chambered heart.

Fact 2: Fish are backboned aquatic animals.

Given the two facts above, answer the question "The two-chambered heart of some backboned aquatic animals is part of their:" with the following options: 
- aerobic capacity
 - three-chambered heart
 - biological diversity
 - it keeps an organism warm
 - the body's largest organ
 - circulatory system
 - salmon
 - swimming system


 

      circulatory system   