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
Intensity of radiation at a distance of 4 cm is R. At the distance of 2 cm, the intensity will be? 
Select and explain. 
A. 1/2 R, B. R, C. 2 R, D. 4 R
D: 4 R
Answer: D: 4 R 
Explanation: The intensity of an X-ray beam (the number of photons per cross sectional area per unit of exposure time) depends on the distance of the measuring device from the focal spot. For a given beam, the intensity is inversely proportional to the square of the distance from the source.
White and Pharoah's Oral Radiology Principles and Interpretation 8th edition