Which of the Following Is Considered the Key Innovation That

Multiple Choice

Which of the following is considered the key innovation that started chordates along the evolutionary path that led to vertebrates?

A) a flexible rod to which muscles are attached,which allowed lateral movement of the back
B) an internal endoskeleton
C) a skin covering the entire body to prevent desiccation
D) a hard shell encasing the body
E) amniotic egg