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
Base population for registered cancer study: 
Select and explain. 
A. 1-2 million, B. 1-3 million, C. 2-5 million, D. 3-7 million
A: 1-2 million
Answer: A: 1-2 million 
Explanation: Ans. a. 1-2 million