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': 'What should be the diameter of tungsten filament used as cathode: \nSelect and explain. \nA. 1mm in diameter., B. 2mm in diameter., C. 1cm in diameter., D. 2cm in diameter.', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]