Perhaps the Greatest Change in the Nature of Aquatic Ecosystems

Perhaps the greatest change in the nature of aquatic ecosystems during Middle Paleozoic resulted from the origin of

A) planktonic invertebrate animals, many of which were predators.
B) nektonic invertebrate animals, many of which were predators.
C) nektonic vertebrate animals, many of which were predators.
D) nektonic invertebrate animals, many of which were herbivores.