respiratory and circulatory digestive and respiratory digestive and urinary urinary and circulatory
I gave my students this multiple choice question: Which two systems are involved when waste and water are removed from blood as it flows through the kidneys?

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

Could you tell me which one is correct?

urinary and circulatory