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: Medicine
Following are sulphonyl urea compounds except: 
Select and explain. 
A. Tolbutamide, B. Chloropropamide, C. Metformin, D. Glibenclamide
C: Metformin
Answer: C: Metformin 
Explanation: None