In a cross between a strain of large guinea pigs and a strain of small guinea pigs, the F1 are phenotypically uniform, with an average size about intermediate between that of the two parental strains. Among 1014 F2 individuals, 3 are about the same size as the small parental strain and 5 are about the same size as the large parental strain. How many gene pairs are involved in the inheritance of size in these strains of guinea pigs?

a. 5

b. 2

c. 3

d. 4