What Is the Most Likely Reason That the Curve in the Top

What is the most likely reason that the curve in the top graph mostly levels off between five and ten species?

A) The prairie community only has five to ten niches.
B) The community is approaching its maximum productivity,given the resources available.
C) With more than 10 species,competition prevents further productivity.
D) With more than 10 species,species diversity no longer influences productivity.
E) The additional species must be smaller and therefore contribute less to the biomass.