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Coronal mass ejections （CMEs） are a major form of solar activities. A CME takes away 10^15-16 g of plasma from solar low corona, to disturb the near-Earth space if the CME direction is favorable. Here we summarize our understandings and reasoning that lead us to conclude that CMEs are the unavoidable products of magnetic helicity accumulation in the corona. Our study puts the formation of magnetic flux ropes and CME eruptions as natural and unavoidable results of coronal evolution.