In the example of the flicker hybrid zone, why does it appear that the flicker hybrids are in a stable hybrid zone?

A) The two kinds of flickers are reproductively isolated from one another.
B) The two kinds of flickers have evolved completely into one distinct group.
C) The two kinds of flickers have shown some reinforcement and thus the hybrids are beginning to disappear.
D) The two kinds of flickers have out-competed their parental populations and exist as an entirely new species.
E) The two kinds of flickers do not show reinforcement, nor have fused into one distinct group and their hybrid zone has not expanded.