skeletal and digestive skeletal and respiratory circulatory and digestive circulatory and respiratory
I gave my students this multiple choice question: What two organ systems are involved in the transfer of oxygen to the cells from outside the body?

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

Could you tell me which one is correct?

circulatory and respiratory