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 Fluoride form that has to be freshly prepared before use is: 
Select and explain. 
A. NaF, B. SnF, C. APF solution, D. APF gel
B: SnF
Answer: B: SnF 
Explanation: None