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': 'Primary teeth have \nSelect and explain. \nA. Thinner root canals than permanent teeth., B. Higher pulp horns than permanent teeth., C. Crown root ratio of 1:4, D. All of the above', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]