excretory and digestive circulatory and excretory digestive and respiratory respiratory and circulatory
I am hesitating between 4 options to answer the following question, which option should I choose?
Question: Oxygen-rich blood returns from the lungs to the heart. Which two systems are most directly involved in this process?
Possibilities:
- excretory and digestive
- circulatory and excretory
- digestive and respiratory
- respiratory and circulatory

respiratory and circulatory