excretory and skeletal systems nervous and endocrine systems circulatory and digestive systems respiratory and circulatory systems
I gave my students this multiple choice question: Which two systems primarily function to rid the human body of the wastes resulting from cellular energy production?

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

Could you tell me which one is correct?

respiratory and circulatory systems