digestive and skeletal skeletal and circulatory circulatory and respiratory respiratory and digestive
I gave my students this multiple choice question: Which two human body systems are most responsible for getting oxygen to cells?

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

Could you tell me which one is correct?

circulatory and respiratory