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': 'Which of the following drug reduces blood pressure primarily by directly decreasing heart rate alone: \nSelect and explain. \nA. Propranolol, B. Prazosin, C. Alpha methyl dopa, D. Nitroprusside sodium', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]