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
Unit of radiation exposure – 
Select and explain. 
A. Rad, B. Rem, C. Gray, D. Roentgen
D: Roentgen
Answer: D: Roentgen 
Explanation: Roentgen is the old unit and coulombs/kg is the new (SI) unit of radiation exposure.