设 R={0,10,20,31,21,32,3}，则关系R的二次合成R○R等于：

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【单选题】【消耗次数：1】

① {0,20,31,3

② {1,02,03,02,13,13,2

③ {0,10,20,31,21,3

④ {2,3}.

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