This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiology, Subject name: Social & Preventive Medicine
[{'content': 'Base population for registered cancer study: \nSelect and explain. \nA. 1-2 million, B. 1-3 million, C. 2-5 million, D. 3-7 million', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. a. 1-2 million', 'role': 'assistant'}]