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
Which of the following orthodontic appliance allows "passive expansion"? 
Select and explain. 
A. Transpalatal arch, B. Quad helix, C. Frankel, D. Rapid maxillary expansion
C: Frankel
Answer: C: Frankel 
Explanation: None