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)}