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': '‘Lead time’ refers to the time between \nSelect and explain. \nA. Disease onset and first critical diagnosis, B. First possible point of detection and usual time of diagnosis, C. Disease onset and first possible point of detection, D. First possible point of detection and final critical point', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]