This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Physics of Radiotherapy, Subject name: Radiology
Unit of radiation exposure: 
Select and explain. 
A. Rad, B. Gray, C. Sievert, D. Roentgen
D: Roentgen
Answer: D: Roentgen 
Explanation: Ans. (d) RoentgenRef: Essentials of radiological Science 2nd ed. 13* Unit of radiation exposure- Roentgen (SI= Coloumb/ kg)* Unit of absorbed radaition- RAD (SI= GRAY)* Unit of biologic effectiveness- Rem (SI= Sievert)