In the Geologic Timescale,an Environmental Condition That Changed Prior to the Appearance

In the geologic timescale,an environmental condition that changed prior to the appearance of complex species,from eukaryote cells to animals and plants,was

A) an "explosion" of simpler and diverse animals that helped develop ecological interrelationships among species.
B) a large increase in atmospheric oxygen gas levels from photosynthesis in prokaryote cells.
C) microevolution had to change the species traits enough that paleontologists could distinguish the geologic eras and periods.
D) non-vascular and vascular plants had to develop the photosynthesis process and produce oxygen gas in the atmosphere.