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
Agent of first choice in an acute attack of Prinzmetals angina is - 
Select and explain. 
A. Diltiazem, B. Nitrates, C. Propranolol, D. Verapamil
B: Nitrates
Answer: B: Nitrates 
Explanation: None