In frogs, a true-breeding female with green skin is crossed to a true-breeding male with yellow skin. All of the F1 offspring have green skin. The F1 female offspring are then crossed to true-breeding males with yellow skin. All of the F2 offspring have green skin. These are observations are consistent with

Select one:
a. maternal effect.
b. genomic imprinting
c. both maternal effect and maternal inheritance.
d. maternal inheritance