Tilman Experimentally Manipulated Plant Species Richness and Measured Total Plant

Tilman experimentally manipulated plant species richness and measured total plant cover after two years. Which statement best describes the relationship between plant species richness and total plant cover that he observed?

A) Plant cover increased with increasing plant species richness but declined at highest species richness.
B) Plant cover increased with increasing plant species richness but leveled off at highest species richness.
C) Plant cover decreased with increasing plant species richness but declined at higher species richness.
D) Plant cover decreased with increasing plant species richness but leveled off at higher species richness.