Demonstrating the Existence of Recognition Alleles Has Proven Quite Difficult

Demonstrating the existence of recognition alleles has proven quite difficult, primarily because

A) green beards occur too frequently to be useful research tools.
B) it is DIFFICULT to eliminate all possible opportunities for learning recognition cues during an animal's lifetime.
C) it is too DIFFICULT to understand the underlying processes that would have to exist to allow them to work.
D) not all species have a major histocompatibility complex (MHC) .