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: Forensic Medicine
Preservative used for blood: UP 10 
Select and explain. 
A. Sodium fluoride, B. Thymol, C. Potassium oxalate, D. No preservative is needed
A: Sodium fluoride
Answer: A: Sodium fluoride 
Explanation: Ans. Sodium fluoride