A population of birds eats nuts and seeds. Some birds have larger beaks, and some have smaller beaks. A pair of birds usually produces four or five offspring each year. After several yeaers of little rain, the seeds became larger and larger. The birds with larger beaks were better at cracking these seeds. After several generations, the average beak size in the population increased. Bird beak size increasing after several generations is: a. decent with modification. b. variation. c. overproduction. d. adaptation