The development of what factored in the development of complex organisms

A. ozone

B. oxygen

C. cyanobacteria

D. all or the above

The development of Ozone, Oxygen, Cyanobacteria factored into the development of complex organisms. The correct option is the last option.

The end of the Paleozoic Era was signaled by mass extinction of land and sea animals. The correct option is the second option.

Reptiles evolved from an ancestor of amphibians that developed an egg with a strong outer shell that kept it from drying out. The correct option is the first option.