An evolutionary model of animal development explains how _____.  

A) animal body plans are constrained by existing structures in ancestral organisms.
B) homeotic genes are not universal among animals.
C) homeotic genes are found only in fruit flies, where they are responsible for odd placement of appendages.
D) new genetic lines are produced less by modification than by unique mutation.
E) somites form as blocks of genes in cells randomly distributed throughout the body.