Why Does Secondary Treatment of Wastewater Require O2

Why does secondary treatment of wastewater require O2?

A) Secondary treatment relies on anaerobic microbes to oxidize organic material.
B) Secondary treatment relies on anaerobic microbes to reduce organic material.
C) Secondary treatment relies on aerobic microbes to oxidize organic material.
D) Photosynthetic bacteria are involved in secondary treatment and they require O2.
E) Flocs require oxygen to facilitate biofilm formation.