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': 'Thickness of lead apron to prevent radiation: \nSelect and explain. \nA. 1 mm, B. 3 mm, C. 0.5 mm, D. 7 mm', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Ans. 0.5 mm', 'role': 'assistant'}]