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': 'Missing cases are detected by: \nSelect and explain. \nA. Active surveillance, B. Passive surveillance, C. Sentinel surveillance, D. Monitoring', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Sentinel surveillance', 'role': 'assistant'}]