A: Cellular respiration reguires oxygen to get the full energy from a carbon source via the process of of the electron transport chain. This requires higher concentrations of oxygen than were available before this time.
For the other two: B did not take advantage of the oxygen as chloroplasts require CO2 and actually produce oxygen and C is a result of the increase of oxygen but it is not cells taking advantage of it, they are simply persisting in environments they used to inhabit, only on a less wide-spread scale.
