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 is \nSelect and explain. \nA. 0.25 mm, B. 0.3 mm, C. 0.4 mm, D. 0.5 mm', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Thickness of lead apron is 0.5 mm.', 'role': 'assistant'}]