This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
Aerobic digestion of sludge takes place in – a)  Trickling filterb) Broad irrigationc) Activated sludged) Septic tank 
Select and explain. 
A. a, B. bc, C. ac, D. b
C: ac
Answer: C: ac 
Explanation: None