What was the outcome of K. F. Koopman's experiment on reproductive isolation between D. pseudoobscura and D. persimilis?

A. The number of interspecific hybrids remained constant over generations
B. The experiment had no effect on reproductive isolation between the two species
C. The number of interspecific hybrids decreased over generations, with hardly any hybrids produced by the tenth generation
D. The number of interspecific hybrids increased over generations
Answer: 