In 1977, Before the Drought on Daphne Major, the Average

In 1977, before the drought on Daphne Major, the average beak depth in the finch population was 9.2 mm. Finches that bred successfully during the drought had an average beak depth of 9.7 mm. The difference between these values represents

A) the response to selection (R) .
B) the selection differential (S) .
C) heritability (H²) .
D) the mutation rate (μ) .