Two Plants Are Crossed, Resulting in Offspring with a 3:1

Two plants are crossed, resulting in offspring with a 3:1 ratio for a particular trait. This suggests

A) that the parents were True-breeding for contrasting traits.
B) incomplete dominance.
C) that a blending of traits has occurred.
D) that the parents were both heterozygous.
E) that each offspring has the same alleles.