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
Which of the following techniques is the best for differentiating recurrence of brain tumour from radiation therapy induced necrosis? 
Select and explain. 
A. MRI, B. Contrast enhanced MR1, C. PET scan, D. CT scan
C: PET scan
Answer: C: PET scan 
Explanation: Ans. PET scan