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': 'Subclinical cases are seen in all except - \nSelect and explain. \nA. Measles, B. Rubella, C. Polio, D. Influenza', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]