This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Introduction to Radiology, Subject name: Radiology
Which is not the 'R' of Radiobiology 
Select and explain. 
A. Repair, B. Redistribution, C. Repopulation, D. Reperfusion
D: Reperfusion
Answer: D: Reperfusion 
Explanation: The Four R's of Radiobiology Four major factors are believed to affect tissue's response to fractionated radiation: Repair of sublethal damage to cells between fractions caused by radiation Repopulation or regrowth of cells between fractions Redistribution of cells into radiosensitive phases of cell cycle Reoxygenation of hypoxic cells to make them more sensitive to radiation