Which of the following statements about the studies on Darwin's finches conducted by Rosemary Grant and Peter Grant since the 1970s is correct?
a) They found no evidence of natural selection acting on beak size.
b) They observed changes in beak size correlated with changes in food availability.
c) They concluded that environmental factors have no impact on the evolution of finches.
d) They focused solely on the mating behaviors of finches.