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 size of actual focal spot is: \nSelect and explain. \nA. 1x3 mm, B. 1x1 mm, C. 1x4 mm, D. 1X2 mm', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]