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 for failure of the seal in PPS area? 
Select and explain. 
A. Under extension, B. Under post damming, C. Over post damming, D. Overextension
A: Under extension
Answer: A: Under extension 
Explanation: None