The Common Ancestors of Birds and Mammals Were Very Early

The common ancestors of birds and mammals were very early (stem) reptiles, which almost certainly possessed 3-chambered hearts (2 atria, 1 ventricle) . Birds and mammals, however, are alike in having 4-chambered hearts (2 atria, 2 ventricles) . The 4-chambered hearts of birds and mammals are best described as

A) structural homologies.
B) vestiges.
C) homoplasies.
D) the result of shared ancestry.
E) molecular homologies.