A Long, Narrow Hybrid Zone Exists in Europe Between the Ranges

A long, narrow hybrid zone exists in Europe between the ranges of the fire-bellied toad and the yellow-bellied toad. The persistence of this zone can be attributed to which of the following factors?

A) Reinforcement strengthens the prezygotic barriers between the two species.
B) Hybrid offspring have the same fitness as nonhybrid offspring.
C) Both species travel long distances over the course of their lives.
D) Individuals from outside the hybrid zone regularly move into the hybrid zone.
E) None of the above