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
Attenuation value (Hounsfield unit) of -1000 represents which of the following? 
Select and explain. 
A. Air, B. Muscle, C. Bone, D. Blood
A: Air
Answer: A: Air 
Explanation: The HU for common substance: SUBSTANCE HU Air -1000 Fat -120 (Range -50 to -120) Water 0 Blood +30 to +45 Muscle +40 Contrast +130 Bone +400 Reference: Essential Radiology By S.Bhadury, 2nd Edition, Page 23