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
The most common cause of failure of the seal in the posterior palatal area is: 
Select and explain. 
A. Under extended distal denture border., B. Overextended distal denture border., C. Distolateral border extended beyond the hamular notch., D. All of the above.
A: Under extended distal denture border.
Answer: A: Under extended distal denture border. 
Explanation: None