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
Traditional unit of radiation exposure is: 
Select and explain. 
A. Air kerma., B. Roentgen., C. Coulomb/kg., D. Gray.
B: Roentgen.
Answer: B: Roentgen. 
Explanation: None