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': 'It is most difficult to perform apically positioned flap\nprocedure in: \nSelect and explain. \nA. Facial surface of incisors, B. Lingual surface of incisors, C. Facial surface of third molars, D. Lingual surface of third molar', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]