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': 'Effective dose in radiation at 1 m is 4 Gy; at 4 m it will be: \nSelect and explain. \nA. 0.25, B. 0.5, C. 2, D. 4', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Intensity of an X-ray beam is inversely proportional to the square of the distance between the source and the point of measure. When the distance from the focal spot is doubled, the intensity of the beam decreases to one quarter.', 'role': 'assistant'}]