The Diatom Asterinonella Excludes the Diatom Cyclotella in Laboratory Experimental

The diatom Asterinonella excludes the diatom Cyclotella in laboratory experimental cultures when phosphate concentrations are low.But,Cyclotella excludes Asterionella when silicate supplies are low.However,when there are balanced supplies of silicate and phosphorus,the two diatoms coexist.This is explained by

A) There being a fundamental niche for both species when each is provided sufficient concentrations of a resource on which they are inferior.
B) There being niche complementarity for both species when each is provided an abundance of a resource.
C) There being no fundamental or realized niche for either species.
D) There being a realized niche for both species when each is provided sufficient concentrations of a resource on which they are inferior.