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': 'Most commonly retained primary tooth is: \nSelect and explain. \nA. Maxillary first molar., B. Mandibular first molar., C. Maxillary lateral incisor., D. Mandibular 2nd molar.', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]