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
In computed tomography (CT), the attenuation value are measured in Hounsefield units (HU). An attenuation value of '0' (zero) HV corresponds to: 
Select and explain. 
A. Water, B. Very dense bone structure, C. AIR, D. Fat
A: Water
Answer: A: Water 
Explanation: A i.e. Water