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': 'Which of the following orthodontic appliance allows "passive expansion"? \nSelect and explain. \nA. Transpalatal arch, B. Quad helix, C. Frankel, D. Rapid maxillary expansion', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]