endocrine and skeletal excretory and nervous circulatory and respiratory digestive and reproductive
I gave my students this multiple choice question: Which two body systems are involved in inhaling oxygen and exhaling carbon dioxide?

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

Could you tell me which one is correct?

circulatory and respiratory