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': 'The most common cause for failure of the seal in PPS area? \nSelect and explain. \nA. Under extension, B. Under post damming, C. Over post damming, D. Overextension', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]