Assume that the length of a type of cucumber at maturity is controlled by three genes (A, B, and C), ...

Assume that the length of a type of cucumber at maturity is controlled by three genes (A, B, and C), each of which has two alleles. The A, B, and C alleles each add 2 inches of cucumber growth, while the a, b, and c alleles add only 1 inch.
 
  A true-breeding 6-inch strain is bred to a true-breeding 12-inch strain. All the F1s produce 9-inch cucumbers. What sizes would you expect to see in the mature F2s? A) Discrete lengths of 6, 9, and 12 inches
  B) Discrete lengths of 6 and 12 inches
  C) Discrete lengths of 9 inches
  D) Discrete lengths of 6, 7, 8, 9, 10, 11, and 12 inches
  E) Lengths ranging continuously from 6 to 12 inches