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
[{'content': 'Which of the following is the most common functioning pancreatic islet cell tumour ? \nSelect and explain. \nA. Gastrinoma, B. Glucagonoma, C. Insulinoma, D. Vipoma', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]