The Mesozoic Era Is Nicknamed the Age of Reptiles and the Cenozoic

The Mesozoic era is nicknamed the Age of Reptiles and the Cenozoic era is nicknamed the Age of Mammals. What do these nicknames mean?

A) They indicate that the two eras are about the same length, because one lineage dominates the fauna of Earth only for a set period of time.
B) They identify the time of greatest diversity of these groups in the fossil record.
C) They identify the point of origin of these groups in the fossil record.
D) Both A and C are true.
E) Both B and C are true.