Answer:
A. mass extinctions.
Explanation:
Mass extinction refers to extinction of species on a big scale usually at the end of geological eras. It paves way for another era and new type of species composition is formed.
Paleozoic era ended with a mass extinction. This was the biggest mass extinction on history of earth and it wiped out 70% of land life and 95% of marine life. It paved way for Mesozoic era which was ruled by the dinosaurs. This era had Triassic, Jurassic and Cretaceous periods. It also ended with a mass extinction called as "The Great Extinction" and Cenozoic era began.
