skeletal and digestive skeletal and respiratory circulatory and digestive circulatory and respiratory
I am hesitating between 4 options to answer the following question, which option should I choose?
Question: What two organ systems are involved in the transfer of oxygen to the cells from outside the body?
Possibilities:
- skeletal and digestive
- skeletal and respiratory
- circulatory and digestive
- circulatory and respiratory

circulatory and respiratory