@JonathanH.Zhangxu I couldn't eliminate D (but ultimately chose AC B), because I used my outside knowledge to assess AC D (bad bad bad!). To me, AC D would imply that biological orders (which is different than species) of an older common ancestor (later epochs) would have more genetic dissimilarities overtime due to natural selection, etc. than biological orders of a more recent common ancestor. I saw it like a tree - a baby tree has very few branches, twigs, and leaves (order, family, genus, species) that are not far from the trunk (common ancestor). However, a large, old tree has many strong branches, with exponentially more twigs and leaves.