In the figure below,the willow tree population leveled off at 475 trees and remained stable in the early 1980s.This most likely occurred because the

A) trees occupied all of the available space that willows were able to occupy.
B) tree seedlings were shaded by the grasses shown in the illustration.
C) population growth rate exceeded the death rate.
D) trees depleted all of the nutrients in the soil and the trees could no longer grow.