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
Flux used in stainless steel solder are all except 
Select and explain. 
A. Borax, B. Boric acid, C. Potassium fluoride, D. Sodium fluoride
D: Sodium fluoride
Answer: D: Sodium fluoride 
Explanation: None