Consider two relations R₁ (A,B) with the tuples(1, 5), (3, 7) and R₂ (A, C) = (1, 7), (4, 9). Assume that R(A,B,C) is the full natural outer join of R₁ and R₂ . Consider the following tuples of the form (A, B, C): a = (1, 5, null), b = (1, null, 7), c = (3, null, 9), d = (4, 7, null), e = (1, 5, 7), f = (3, 7, null), g = (4, null, 9). Which one of the following statements is correct?