Refer to the Figure

Refer to the figure.
What is the best description of the relationship identified by van der Heijden et al. between the average shoot biomass of the target plants and the number of mycorrhizal fungal species associated with them?

A) When the number of species is small, shoot biomass is also low; as the number of species increases, shoot biomass also increases.
B) When the number of species is small, shoot biomass is low; as the number of species increases, shoot biomass rapidly increases, then begins to level off.
C) When the number of species is small, shoot biomass is low; when the number of species is moderate, shoot biomass is high; when the number of species is high, shoot biomass is low.
D) The number of species has no effect on shoot biomass.