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': 'In a controlled trial to compare two treatments, the main purpose of randomization is to ensure that - \nSelect and explain. \nA. The two groups will be similar in prognostic factors, B. The clinician does not know which treatment the subjects will receive, C. The sample may be referred to a known population, D. The clinician can predict in advance which treatment the subjects will receive', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]