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: Medicine
[{'content': 'SARS causative agent - \nSelect and explain. \nA. Corona-virus, B. Picorna-virus, C. Myxovirus, D. Retrovirus', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]