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': 'Post exposure active immunization can be done fora)  Rabiesb)  Measlesc)  Cholerad)  Yellow fever \nSelect and explain. \nA. da, B. ab, C. da, D. ac', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]