Which statement describes a finding of Wilson and Tilman's studies of the grass Schizachyrium scoparium?

A) The plants compete only for aboveground resources.
B) The plants compete only for belowground resources.
C) The plants compete for both aboveground and belowground resources, and competition is most intense for whichever resources are scarcer.
D) The plants compete for both aboveground and belowground resources, and competition is most intense for whichever resources are more abundant.