This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Radiotherapy, Subject name: Radiology
The radiation unit used to for effective dose is: 
Select and explain. 
A. Rad, B. Rem, C. Sievert, D. Gray
C: Sievert
Answer: C: Sievert 
Explanation: Ans. C. SievertQuantityOld unitNew SI unitActivityCurieBecquerelExposureRoentgenCoulomb/kgAbsorbed doseRadGrayDose equivalentRemSievert