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: Medicine
[{'content': 'In a patient with chronic atrial fibrillation with a regular beat of 60/min , the most probable cause is - \nSelect and explain. \nA. Sleep, B. Digitalis toxicity, C. Sino nodal block, D. Hypothyroidism', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]