Consider the Figure Below

Consider the figure below. The bars show changes in primary productivity with and without endomychorrhizal associations for a group of plants that diverged early in the history of terrestrial plants. Which of the following statements supports the data shown in the figure?

A) Endomycorrhizal associations do not increase primary productivity in these plants.
B) Endomycorrhizal associations increase primary productivity in these plants.
C) Endomycorrhizal associations only help these plants at high CO₂ concentrations.
D) Endomycorrhizal associations only help these plants at low CO₂ concentrations.