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': 'Initial rapid repolarization in cardiac muscle is mediated through: \nSelect and explain. \nA. Opening of K+  channel, B. Opening of Ca+ channel, C. Opening of Na+ channel, D. Closing of Na+ channel', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Opening\xa0 of\xa0 K+\xa0channel\xa0>>\xa0Closing of Na channel\xa0\nThe\xa0 main\xa0 factor\xa0 is\xa0 opening\xa0of\xa0K+\xa0channel.\xa0\nAlthough\xa0 Ganong has\xa0 maintained\xa0 both closure\xa0 of\xa0 Na+\xa0channels\xa0 and\xa0 opening\xa0of one\xa0 type\xa0 of\xa0 K+\xa0channel\xa0and\xa0closure\xa0of\xa0Na channel only,\nThe recent concept is K+\xa0channel opening is the main reason of phase 1.\xa0\n\nPhase\xa0 0\xa0 (depolarization),\xa0 fast\xa0 sodium\xa0 channels\xa0 open.\nPhase 1 (initial repolarization), fast sodium channels close.\xa0\nPhase\xa0 2\xa0 (plateau),\xa0 calcium\xa0 channels\xa0 open\xa0 and\xa0 fast potassium\xa0 channels\xa0 close.\nPhase 3 (rapid repolarization), calcium channels close and slow potassium channels open.\nPhase 4 (resting membrane potential) averages about −90 millivolts.', 'role': 'assistant'}]