This topic studies the sequence of metal activities and its application. When judging the strength of metal activity, it can be determined by the speed or intensity of reaction between different metals and another substance. When reacting with the same substance, the stronger the phenomenon, the stronger its activity. From the reactions of A, B and C with air, we can know that A has the strongest reaction and the strongest activity with air. From the phenomenon that A, B and C react with oxygen, C burns violently, and B still turns black, so C reacts more violently than B, so C is more active than B, so it can be determined that the order of the three activities from strong to weak is: A, C and B, so A is chosen.