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 not used in diagnosis of insulinoma - \nSelect and explain. \nA. 72 hours fasting blood glucose levels, B. C peptide levels, C. Insulin/glucose ratio, D. D-xylose test', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]