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': 'Quality of the X-ray beam is governed by: \nSelect and explain. \nA. kVp, B. mA, C. Filament current, D. Length of the X-ray tube', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]