Beginning About 500 Million Years Ago, Plants Began to Colonize

Beginning about 500 million years ago, plants began to colonize land.One factor that made this colonization possible was the presence of an atmosphere with high concentrations of carbon dioxide, which was necessary for photosynthesis.This suggests that before colonization, the green aquatic algae that eventually evolved into plants were prevented from forming land communities by

A) competition from other plants.
B) competition from other aquatic algae.
C) lack of root systems.
D) lack of a strong species pool.
E) abiotic factors such as environmental gases.