In the Example of Darwin's Finches,there Were Changes in Beak

In the example of Darwin's finches,there were changes in beak size because

A) some finches changed their beak size in response to environmental changes.
B) environmental variables directly affected how large beaks would grow.
C) finches with larger beaks had an advantage in obtaining food during an environmental crisis.
D) a mutation occurred that gave the finches a needed change in beak size.