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: Dental
[{'content': 'Not a contributing factors for ANUG \nSelect and explain. \nA. Pericoronal flap, B. Poor oral hygiene, C. Smoking, D. Apthous ulcers', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]