In vertebrates such as a chicken, a mouse, of a python, variation in the number of vertebrae in the spine is determined by Hox genes. Which MOST LIKELY way that Hox genes account for this variations?

A) As Vertebrates evolved, mutations in Hox genes led to new vertebral organization during embryological development

B) As vertebrates evolved, mutations in Hox genes directly resulted in new proteins that determined vertebrae

C)Long before vertebrates evolved, a wide variety of Hox genes developed and many codes for vertebral structures

D) Environmental factors, which differ among vertebrate groups, activate specific Hox genes that code for specific structures