Refer to the Figure

Refer to the figure.
In the study by van der Heijden and colleagues comparing the effects of differing numbers of mycorrhizal species (0?-14) on the same 15 plant species, the shoot biomass of the target plants was found to

A) increase steadily with the number of mycorrhizal fungal species.
B) increase at first and then level off as the number of mycorrhizal fungal species increased.
C) increase at first but then drop sharply as the number of mycorrhizal fungal species increased.
D) be high when only a few mycorrhizal fungal species were present and then decrease as more were added.