This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Pancreas, Subject name: Pharmacology
Which of the following oral hypoglycemic agent is preferred in obese patients? 
Select and explain. 
A. Tolbutamide, B. Glipizide, C. Gliclazide, D. Metformin
D: Metformin
Answer: D: Metformin 
Explanation: Metformin doesn't cause weight gain. It may rather cause weight loss. So, it is preferred in obese patients.