Two Plant Species Live Along a Gradient, from a Rich

Multiple Choice

Two plant species live along a gradient, from a rich temperate valley floor to halfway up a mountain slope.In the valley, the two plants are competitors, but going up the gradient their relationship changes and becomes more and more commensal.This change in their relationship is most likely the result of

A) greater predation.
B) greater food availability.
C) an increasingly stressful climate.
D) a decreasingly stressful climate.
E) less space, requiring closer growth habits.