How do we know that the xylem and phloem of a plant are similar to the veins and arteries of a human?

a. we observe and record both organisms under many similar conditions
b. we assume that they both serve the same function because they are the outer covering.
c. we just know that the xylem and phloem transport blood through the tree.
d. we count the number of tree rings to see how old the tree is.