skeletal system immune system endocrine system reproductive system
I gave my students this multiple choice question: Michael has to write a paper on the formation of red blood cells. In addition to the circulatory system, what other body system should he research to find information about the formation of red blood cells?

Only one answer is correct among these 4 choices:
- skeletal system
- immune system
- endocrine system
- reproductive system

Could you tell me which one is correct?

skeletal system