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
[{'content': 'Preservative used for blood: UP 10 \nSelect and explain. \nA. Sodium fluoride, B. Thymol, C. Potassium oxalate, D. No preservative is needed', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. Sodium fluoride', 'role': 'assistant'}]