The Level of Atmospheric CO₂ Has Varied Considerably Over the Years

The level of atmospheric CO₂ has varied considerably over the years. Currently, the level of atmospheric CO₂

A) is four times what is was during the time of the dinosaurs.
B) favors C₄ plants under hot conditions.
C) has resulted in maximum CO₂ fixation by rubisco.
D) is decreasing.
E) prevents the occurrence of photorespiration.