What Does the Competitive Exclusion Principle State

What does the Competitive Exclusion Principle state?

A) The Competitive Exclusion Principle states that if two competing species coexist in a stable environment, then they do so as a result of differentiation of their realized niches.
B) The Competitive Exclusion Principle states that all species within a community coexist in an unstable environment, and that they are structured by competition that leads to realized niches.
C) The Competitive Exclusion Principle states that if a guild of competing species coexist in an stable environment, they do so as a result of differentiation of their fundamental niche.
D) None of the above