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
Not a contributing factors for ANUG 
Select and explain. 
A. Pericoronal flap, B. Poor oral hygiene, C. Smoking, D. Apthous ulcers
D: Apthous ulcers
Answer: D: Apthous ulcers 
Explanation: None