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': 'Commonly used collimating device \nSelect and explain. \nA. Aluminium filter, B. Lead diaphragm, C. Molybdenum cup, D. Tungsten filament', 'role': 'user'}, {'content': 'Answer: B \nExplanation: The collimating device often used is a lead diaphragm with a circular aperture for circular PIDs or a rectangular aperture for rectangular PIDs and the lead-lined PID.\xa0\nFrommer’s Radiology for Dental Professional, pg-22', 'role': 'assistant'}]