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': "Sildenafil's mechanism of action can be best described as: \nSelect and explain. \nA. β–adrenoceptor blocking agent, B. Selective inhibitor of phosphodiesterase type 5, C. Inhibits reuptake of both serotonin and nor adrenaline, D. Selective serotonin reuptake inhibitor", 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]