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': 'Beta blockers are antiarrhythmic agents of class \nSelect and explain. \nA. 1, B. 11, C. 111, D. 1V', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Refer kDT 6/e p 511 The five main classes in the Vaughan Williams classification of antiarrhythmic agents are: Class I agents interfere with the sodium (Na+) channel. Class II agents are anti-sympathetic nervous system agents. ... Class III agents affect potassium (K+) efflux. Class IV agents affect calcium channels and the AV node.', 'role': 'assistant'}]