This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Special topics, Subject name: Forensic Medicine
[{'content': 'Never used in chemical analysis \nSelect and explain. \nA. Saturated NaCl solution, B. Formalin, C. Rectified spirit, D. Sodium fluoride', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Formalin cannot be used for preservation because it hardens and fixes the tissues and poison cannot be extracted. Ref: FORENSIC MEDICINE AND TOXICOLOGY Dr PC IGNATIUS THIRD EDITION PAGE 404', 'role': 'assistant'}]