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: Physiology
[{'content': 'Action potential across the cell membrane is maintained by? \nSelect and explain. \nA. Na + K+  pump, B. cAMP, C. Ca ++, D. Phosphodiesterase', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]