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': 'An example of unilateral fixed space maintainer is: \nSelect and explain. \nA. Crown and loop space maintainer, B. Nance palatal arch, C. Lingual arch space maintainer, D. Transpalatal arch', 'role': 'user'}, {'content': 'Answer: A \nExplanation: All other options are bilateral space maintainers.', 'role': 'assistant'}]