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
What is the S.I. unit of effective dose? 
Select and explain. 
A. Becquerel, B. Sievert, C. Gray, D. Roentgen
B: Sievert
Answer: B: Sievert 
Explanation: The effective dose (E) is used to estimate the risk in humans. It is hard to compare the risk from a dental exposure with, for example, the risk from a radiographic chest examination because different tissues with different radiosensitivities are exposed. 
The unit of effective dose is the Sv.