Researchers are developing a mathematical function model h to estimate the height of any tree, in meters, based on the tree's circumference c, in meters, as measured at a height of one meter from the base of the tree. Which of the following statements is an assumption about what is consistent in this context?
a) The height of a tree can be accurately estimated using its circumference
b) The height of a tree is directly proportional to its circumference
c) The height of a tree decreases as its circumference increases
d) The height of a tree is independent of its circumference
