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
CT number of water and bones respectively 
Select and explain. 
A. 100,0, B. 0,-1000, C. 0, +1000, D. +1000,-100
C: 0, +1000
Answer: C: 0, +1000 
Explanation: Answer: c) 0, + 1000CT- scan or CAT scanInvented by God Frey Hounsfield in 1963, awarded Nobel Prize.Basic principle of CT is linear attenuation of x-rays.Incident x-rays are linearly attenuated by their interaction with orbital electrons of tissues.Measurement of attenuation of emerging / detected beam - gives density of intervening tissues and this density forms basis of signal intensity variation obtained in x-ray tomograms.Two type of contrast agents are usedIonic - water soluble iodide dyes (e.g. Na- diatrizoate, megliumine, conray, urograffin, angiografin) - may cause anaphylaxisNon ionic- Safer but expensive e.g. iohexol (Omnipaque), lopamiroElectron density of tissues in numbered as Hounsfield number (H.N.) or Hounsfield units.Air have a value of -1000 HUFat:-120 to-200 HUWater-0 HUSoft tissue: 20-60 HUBlood: 50-60 HUBone:+1000 HU