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': 'Crown completion of Maxillary 2nd premolar occur \nSelect and explain. \nA. At birth, B. 1 ½ year, C. 2 ½ to 3 year, D. 6-7 years', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]