You are presented with the question "what have a relatively complex circulatory system with three-chambered heart?" and the following answer choices: 
- sharks
 - humans
 - bacteria
 - Birds
 - blood
 - frogs
 - salmon
 - pacemaker

Now knowing that amphibians have a relatively complex circulatory system with a three-chambered heart and all frogs are amphibians, choose the best answer.


 

      frogs   