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': 'Agent of first choice in an acute attack of Prinzmetals angina is - \nSelect and explain. \nA. Diltiazem, B. Nitrates, C. Propranolol, D. Verapamil', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]