This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Heart, Circulation, and Blood, Subject name: Physiology
[{'content': 'Action potential in cardiac muscles is due to which ions- \nSelect and explain. \nA. K+, B. Na+, C. Ca+2, D. C1-', 'role': 'user'}, {'content': "Answer: B \nExplanation: Ans. is 'b > a & c' i.e., Na+ > K+ & Ca+2 * Na+, K+ and Ca+2, all are involved in full cycle of action potential in cardiac muscle. But, the main phase (phase 0 or depalarization) is due to Na+ ions.* Phase 0 (phase of rapid depolarization) - opening of fast sodium channels with Na+ influx.* Phase 1 (initial phase of rapid repolarization) - closure of fast sodium channels.* Phase 2 (plateu phase) - opening of voltage gated slow Ca++ channels with calcium influx.* Phase 3 (final repolarization) - opening of K+ channels with K+ efflux.* Phase 4 - Resting membrane potential.", 'role': 'assistant'}]