Refer to the Figure

Refer to the figure.
In the study by van der Heijden et al. of mycorrhizal fungi, the root biomass of the target plants (shown in the figure) was found to

A) increase at first and then level off as the number of fungal species was increased.
B) increase steadily as the number of fungal species was increased.
C) be highest when the fungal species were at an intermediate level.
D) be high when there were only a few fungal species present and then decrease as more were added.