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
In a patient with chronic atrial fibrillation with a regular beat of 60/min , the most probable cause is - 
Select and explain. 
A. Sleep, B. Digitalis toxicity, C. Sino nodal block, D. Hypothyroidism
B: Digitalis toxicity
Answer: B: Digitalis toxicity 
Explanation: None