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. 1 mm in diameter, B. 2 mm in diameter, C. 1 cm in diameter, D. 2 cm in diameter', 'role': 'user'}, {'content': 'Answer: B \nExplanation: The cathode\xa0 in an x-ray tube consists of a filament and a focusing cup. The filament is the source of electrons within the x-ray tube. It is a coil of tungsten wire about 2 mm in diameter and 1 cm or less in length. Filaments typically contain about 1% thorium, which greatly increases the release of electrons from the heated wire.\nRef: Oral Radiology, Principles and Interpretation / Stuart C. White, Michael J. Pharoah - 7th ed - pg - 4', 'role': 'assistant'}]