The ancestors of modern-day fish were vertebrate chordates. Vertebrate chordates are believed to have evolved from invertebrate chordates, which were similar to modern-day tunicates. What change must have occurred during the evolution from invertebrate chordates to vertebrate chordates? A. An endoskeleton evolved to enclose the spinal cord, which develops from the notochord. B. An endoskeleton evolved to enclose the dorsal nerve cord, which develops from the notochord. C. A backbone evolved to enclose the notochord, which develops from the spinal cord. D. A backbone evolved to enclose the spinal cord, which develops from the dorsal nerve cord.