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
Curie is unit of: 
Select and explain. 
A. Radiation exposure, B. Radiation absorption, C. Radioactivity, D. All of the above
C: Radioactivity
Answer: C: Radioactivity 
Explanation: C i.e. Radioactivity