Birds that fly have lightweight bones some of which are hollow how might this unique body structure benefit the bird?

Need asap plz hurry im being timed when a body cell divides through the process of mitosis, the chromosomes in the daughter cells a. represent only the healthiest chromosomes from the parent cell. b. represent only half of the chromosomes in the parent cell. c. are identical to the chromosomes of the parent cell. d. are formed when chromosomes from the parent cell cross over.