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': 'Quinidine exerts its action on heart by: \nSelect and explain. \nA. Ca2+ channel blockade, B. Na+ channel blockade, C. K+ channel opening, D. Cl– channel opening', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]