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': 'In primary impression the stock tray should be ________ mm \nlarger than ridge. \nSelect and explain. \nA. 2 mm, B. 4 mm, C. 6 mm, D. 8 mm', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]