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': 'Eruption of Deciduous Mandibular 1st molar occur at \nSelect and explain. \nA. 8-9 months, B. 19-21 months, C. 19-20 months, D. 12-14 months', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]