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': 'Best space maintainer is: \nSelect and explain. \nA. Active space maintainer, B. Passive space maintainer, C. Band and loop, D. Pulpotomised primary tooth', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]