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': 'Randomization means - \nSelect and explain. \nA. Mixing of cases and controls, B. Selecting cases, C. Selecting controls, D. Vaccine and Erythromycin', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]