Male Mallards Have Had Emerald Green Head Feathers and Wings

Male mallards have had emerald green head feathers and wings with metallic blue patches for hundreds of years, whereas female mallards have been drab, brown-feathered ducks.This phenotypic situation suggests that mallards may be an example of

A) directional selection.
B) polyploidy.
C) allopatric speciation.
D) disruptive selection.
E) sexual selection.