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
[{'content': 'The heart of activated sludge process is \nSelect and explain. \nA. Secondary sedimentation tank, B. Primary sedimentation, C. Digestion tank, D. Aeration tank', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]