9[单选题]
设有关系R和S，与R-S等价的元组关系演算表达式是( )

A.{t|R(t)∨┐s(t)}

B.{t|R(t)∧┐S(t)}

C.{t|(u)R(t∨┐S(u))

D.{t| (u)R(t)∧┐S(u)}