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': 'Which of the following phase is absent in action potential of pacemaker cells? \nSelect and explain. \nA. Phase 0, B. Phase 1, C. Phase 3, D. Phase 4', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]