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': 'Periodontal flap surgery is most difficult in: \nSelect and explain. \nA. Incisors (facially), B. Incisors (lingually), C. 2nd molars (facially), D. 2nd molars (distally)', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]