Fact 1: Amphibians have a relatively complex circulatory system with a three-chambered heart.

Fact 2: All frogs are amphibians.

Given the two facts above, answer the question "Frogs have a _-chambered heart." with the following options: 
- alveoli
 - organ
 - ovaries
 - oxygen
 - edible
 - three
 - nektar
 - CO 2


 

      three   