设有两个关系R(A,B)和S(B,C),与下列SELECT语句SELECT A,B FROM R JOIN S ON R.B=S.B WHERE C≠'C56' 等价的关系代数表达式是( )
πA,B (σC≠'C56' and R.B=S.B(R∞S))
πA,B (R ∞ S)
R-πA,B (σC= 'C56'(R∞S))
R-πA,B (σC≠'C56'(R∞S))
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