excretory and digestive circulatory and excretory digestive and respiratory respiratory and circulatory
I gave my students this multiple choice question: Oxygen-rich blood returns from the lungs to the heart. Which two systems are most directly involved in this process?

Only one answer is correct among these 4 choices:
- excretory and digestive
- circulatory and excretory
- digestive and respiratory
- respiratory and circulatory

Could you tell me which one is correct?

respiratory and circulatory