During Secondary Aerobic Wastewater Treatment, Some of the Activated Sludge

During secondary aerobic wastewater treatment, some of the activated sludge goes________ while the rest goes________ .

A) back to the aerator / to the anoxic sludge digestor
B) to the anoxic digestor / back to the primary system
C) back to the primary system / to the aerator
D) back to the primary system / to the tertiary digestor