A long, narrow hybrid zone exists between the ranges of the fire-bellied toad and the Yellow-bellied toad in Europe. Which of the following is a factor contributing to the Persistence of this zone?

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.