We surveyed the social breeding behavior of the African bee-eater on videotape. This survey displayed several conspicuous phenomena. On the one hand, older siblings (from a previous breeding year) actively helped to feed their newly hatched young chick siblings, even at the costs of these older siblings failing to produce chicks of their own. On the other hand, in contrast, these three newly hatched chicks competed violently with one another, ultimately resulting in two of the chicks perishing. Which of the following is (are) a true statement about these strongly contrasting social relationships between different siblings?

a. The genetic costs to one young chick of succumbing to competition from a 50% related second chick are greater than the benefits from allowing the second chick to survive at the first chick's expense.

b. The older chicks have already demonstrated their fitness by survival; however, for the good of the bee-eater species, it is essential that the newly hatched chicks compete to demonstrate who is fitter.

c. The genetic costs to older siblings of feeding their 50% related younger siblings are substantially outweighed by the genetic replication benefit to the older siblings through the survival and reproduction of the younger siblings.

d. all of the above

e. a. and c. f. b. and c.