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
What should be the diameter of tungsten filament used as cathode: 
Select and explain. 
A. 1mm in diameter., B. 2mm in diameter., C. 1cm in diameter., D. 2cm in diameter.
B: 2mm in diameter.
Answer: B: 2mm in diameter. 
Explanation: None