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': 'The most common cause of chronic bronchitis is: \nSelect and explain. \nA. Air pollution, B. Septic fibrosis, C. Smoking, D. Recurrent aspiration pneumonia', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]