This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardiovascular system, Subject name: Pharmacology
[{'content': 'Drug directly acting on blood vessels is: \nSelect and explain. \nA. Hydralazine, B. Verapamil, C. Propranolol, D. Methyaldopa', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ref:KDT 6/e p547 Some drugs used to treat hypeension, such as calcium channel blockers -- which prevent calcium from entering blood vesselwalls -- also dilate blood vessels. But the vasodilators that work directly on the vessel walls are hydralazine and minoxidil.', 'role': 'assistant'}]