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': 'When taking a reline impression for an upper denture,\nwhich is the most common error? \nSelect and explain. \nA. Failure to remove undercuts, B. Improperly rounded peripheries, C. Improper establishment of a post dam area, D. Changed centric occlusion', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]