Refer to the Figure

Refer to the figure.
In the study illustrated in the figure, van der Heijden and colleagues compared the effects of differing numbers of mycorrhizal species (0?-14) on the same 15 plant species. They measured the effects of this mycorrhizal species diversity on net primary production (measured as shoot and root biomass) and on cycling of nutrients (measured as phosphorus content) . Based on the figure, what can be concluded about the effect of mycorrhizal diversity on plant ecosystems?

A) All three factors decrease, indicating that mycorrhizal diversity inhibits plant growth.
B) All three factors increase, indicating that mycorrhizal diversity improves plant growth.
C) There appears to be little change in any of the three factors; thus, mycorrhizal diversity has little effect on plant growth.
D) Shoot biomass and phosphorus content increase, but root biomass shows little change, indicating that the presence of mycorrhizae in soil depresses root growth.