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
The iron carbide in orthodontic wire is in the form of: 
Select and explain. 
A. Martensite, B. Ferrite, C. Carbide, D. Austenite
D: Austenite
Answer: D: Austenite 
Explanation: None