This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Fundamentals in Radiology, Subject name: Radiology
Hounsfield unit is zero for which of the following? 
Select and explain. 
A. Air, B. Water, C. Fat, D. Dense bone
B: Water
Answer: B: Water 
Explanation: Hounsfield units are used to measure the X ray attenuation in CT scan. Depends on absorption/attenuation coefficient. Water is assigned a HU value of 0.