You are given a science question (easy-level) and four answer options (associated with "A", "B", "C", "D"). Your task is to find the correct answer based on scientific facts, knowledge, and reasoning. Do not generate anything else apart from one of the following characters: 'A', 'B, 'C', 'D'. There is only one correct answer for each 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?
(A) the skeletal system (B) the endocrine system (C) the respiratory system (D) the integumentary system
A