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: Radiology
[{'content': 'The substance most commonly used for protection\nagainst X-ray radiation is \nSelect and explain. \nA. Zinc, B. Steel, C. Lead, D. porcelain', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]