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: Pharmacology
[{'content': 'The primary endogenous substrate for nitric oxide synthase (NOS) is : \nSelect and explain. \nA. Citrulline, B. Arginine, C. Heme, D. Methionine', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]