Answer
a.
Work Step by Step
After the dinosaurs were wiped out in a mass extinction at the end of the Cretaceous period, which also marked the end of the Mesozoic era, mammals began to flourish. During the time of the dinosaurs, mammals were limited in diversity and small in size. The Cenozoic era, which succeeded the Mesozoic era, saw the rise of the mammals. By the Eocene epoch, all current orders of mammals had evolved.
