21. Gregor used parent generation seeds that have yellow-round phenotype (genotype YYRR) to cross with the parent generation seeds that have green-wrinkled phenotype (genotype yyrr). Then he crossed their F1 seeds to generate F2 seeds. Given the gene encodes color and the gene encodes shape of the seeds are independent assorted, what is the expected phenotypic ratio of the F2 seeds (Yellow-Round: Green-Round: Yellow-Wrinkled: Green-Wrinkled)?
(A) 9:3:3:1
(B) 1:2:2:1
(C)2:3:3:2
(D) 4:1:1:4
(E) 1:4:4:1