All of Darwin's Finches Came from a Single Species with a "Normal

All of Darwin's finches came from a single species with a "normal" beak.Which factor can explain the development of the variety of beaks seen in these finches today?

A) Different mutations in beak shape occurred in different populations of finches.
B) Different food sources on each island created selection pressure for different beak shapes on each island.
C) Genetic drift resulted in different islands having different allele frequencies for the genes controlling beak shape.
D) All of these factors help explain the variety of beaks.
E) None of these explain the variety of beaks.