With the 14 Species of Galapagos Finches, What One Key

With the 14 species of Galapagos finches, what one key adaptation allowed them to share resources and not outcompete each other?

A) Changes in beak size allowed them to each use a different food resource.
B) Changes in wing size altered flight speed so they could get away from each other.
C) It wasn't so much an adaptation as Charles Darwin moving each species to a separate island.
D) Whenever a population dies off on one island, replacements move in from another so no one ever truly goes extinct.
E) They migrate whenever competition gets too stiff.