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': 'Digitalis acts in CHF by: \nSelect and explain. \nA. Na+ K+ ATPase inhibition, B. Na+ K+ ATPase stimulation, C. Blockade of calcium channels, D. Increasing the refractory period of AV node', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]