the skeletal system the endocrine system the respiratory system the integumentary system
I gave my students this multiple choice question: Xylem cells have very thick walls. They make up most of a tree trunk and its branches. What system in vertebrates provides the same function as the xylem cells in a tree?

Only one answer is correct among these 4 choices:
- the skeletal system
- the endocrine system
- the respiratory system
- the integumentary system

Could you tell me which one is correct?

the skeletal system