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': 'Which primary tooth generally erupts last? \nSelect and explain. \nA. Maxillary second molar., B. Mandibular second molar., C. Maxillary canine., D. Mandibular canine.', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Textbook chronologies indicate that the maxillary primary second molar is most commonly the last\xa0 to erupt:\n\n24 months (Woelfel and Scheid, 2002)\n20 to 30 months (Kraus et al., 1969)\n25 to 33 months (Ash and Nelson, 2003)', 'role': 'assistant'}]