According to Geological Evidence and Computer Models Based Upon That

According to geological evidence and computer models based upon that evidence,which statement about the concentration of oxygen in our atmosphere is correct?

A) No oxygen was present prior to the Cambrian;oxygen levels have uniformly risen since and today's atmosphere is more oxygen rich than at any time in Earth's history.
B) Oxygen levels were greatest during the Cambrian but have been steadily declining ever since;today's atmosphere is less oxygen rich than at any time in Earth's history since the Precambrian.
C) Oxygen levels have generally risen during the Phanerozoic but peaked during the Mesozoic ("age of dinosaurs") and declined sharply in the Early Cenozoic.
D) Oxygen levels have generally risen during the Phanerozoic but peaked during the Late Paleozoic ("age of coal swamp forests") and declined sharply in the Early Mesozoic.