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
Periodontal flap surgery is most difficult in: 
Select and explain. 
A. Incisors (facially), B. Incisors (lingually), C. 2nd molars (facially), D. 2nd molars (distally)
D: 2nd molars (distally)
Answer: D: 2nd molars (distally) 
Explanation: None