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': 'The diameter and length of the component marked with arrow is: \nSelect and explain. \nA. 3 mm and 2 cm, B. 2 cm and 3 mm, C. 2 mm and <1 cm, D. 4 mm and <1 cm', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Cathode:\nThe cathode 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 approximately 2 mm in diameter and 1 cm or less in length. Filaments typically contain approximately 1% thorium, which greatly increases the release of electrons from the heated wire. The filament is heated to incandescence with a low-voltage source and emits electrons at a rate proportional to the temperature of the filament.', 'role': 'assistant'}]