Why was the founder species of finches on the galapagos island able tonfor such a large variation of beak types

if these are your options C is the answer

A) The birds colonizing the islands were reproductive isolated. 
B) There were many varieties of finches that landed on the island. 
C) They mated with other species of finches that were already present on the islands. 
D) The new beak types allowed for faster mating.

Ithink due to shortage of water