As Cyanobacterial Populations Boomed, the Process of Photosynthesis Generated Significant

As cyanobacterial populations boomed, the process of photosynthesis generated significant levels of O₂ in the oceans and atmosphere.Which statement accurately describes conditions surrounding the evolution of eukaryotes and the early role of the proteobacterial endosymbiont that would eventually develop into a mitochondrion?

A) The increase in O₂ resulted in an increase in the energy required to maintain the eukaryotic cell—the phagocytosis of a proteobacterium was originally intended as a form of food storage.
B) The increase in O₂ resulted in a population boom for eukaryotes—those that enlisted the aid of a proteobacterial endosymbiont were more likely to find food sources than those that did not.
C) The increase in O₂ had a reducing effect on most eukaryotes, and the acquisition of a proteobacterium that could reduce this O₂ into water allowed the eukaryotic host to survive.
D) The increase in O₂ had did not have an effect on most eukaryotes, but the acquisition of a proteobacterium that coupled O₂ reduction with ATP production provided an additional source of energy to the eukaryotic host.
E) The increase in O₂ had an oxidizing effect on most eukaryotes, and the acquisition of a proteobacterium that could reduce this O₂ into water allowed the eukaryotic host to survive.