In the Case of Darwin's Finches,an Ancestral Finch Species from the Mainland

In the case of Darwin's finches,an ancestral finch species from the mainland arrived on the Galápagos Islands and soon developed into many new species via adaptive radiation.The finches did NOT undergo adaptive radiation back on the mainland.What is the most plausible biological explanation?

A) Directional selection works better on islands.
B) Competition from many other bird species on the mainland provided stabilizing selection that was absent on the islands.
C) The environment on the mainland was completely uniform.
D) The founder effect greatly expanded the variation in alleles in the Galápagos finch gene pool.
E) The ancestral mainland finch was reproductively isolated.