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
The heart of activated sludge process is 
Select and explain. 
A. Secondary sedimentation tank, B. Primary sedimentation, C. Digestion tank, D. Aeration tank
D: Aeration tank
Answer: D: Aeration tank 
Explanation: None