George’s cat had a litter of seven kittens. four of the kittens are male. three are female. which set of symbols correctly describes the gender of the kittens?

a. xx, xx, xx, xx, xy, xy, xy

b. xy, xy, xy, xy, xx, xx, xx

c. y, y, y, y, x, x, x

d. x, x, x, x, y, y, y