A Narrow Hybrid Zone Separates the Toad Species Bombina Bombina

A narrow hybrid zone separates the toad species Bombina bombina and Bombina variegata. What is true of those alleles that are unique to the parental species?

A) Such alleles should be absent.
B) Their allele frequency should be nearly the same as the allele frequencies in toad populations distant from the hybrid zone.
C) The alleles' heterozygosity should be higher among the hybrid toads there.
D) Their allele frequency on one edge of the hybrid zone should roughly equal their frequency on the opposite edge of the hybrid zone.