单项选择题

 Let R = {(1,2), (2,3), (3,3)} and S={(1,1), (3,3)} be two relations. R◦S-S◦R =(    ).
A、{(1, 2), (2, 3)}
B、{(2, 3)}
C、{(1, 2)}
D、\Phi

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