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': 'Which of the following vaccines can be used: \nSelect and explain. \nA. 1 and 2 alone, B. 1 alone, C. 3 and 4 alone, D. 2 alone', 'role': 'user'}, {'content': 'Answer: A \nExplanation: ANS. A', 'role': 'assistant'}]