设一个关系模式为R(A,B,C)，对应的关系内容为R={{1,10,50}, {2,10,60}, {3,20,72}, {4,30,60}}，则δB15(R)的运算结果中具有的元组个数为（）。

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【单选题】【消耗次数：1】

① 1

② 2

③ 3

④ 4

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