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
The size of actual focal spot is: 
Select and explain. 
A. 1x3 mm, B. 1x1 mm, C. 1x4 mm, D. 1X2 mm
A: 1x3 mm
Answer: A: 1x3 mm 
Explanation: None