Consider two relations $R_1(A,B)$ with the tuples $(1,5), (3,7)$ and $R_2(A,C) = (1,7),(4,9)$. Assume that $R(A,B,C)$ is the full natural outer join of $R_1$ and $R_2$. 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?
- $R$ contains $a, b, e, f, g$ but not $c, d$.
- $R$ contains all $a, b, c, d, e, f, g$.
- $R$ contains $e, f, g$ but not $a, b$.
- $R$ contains $e$ but not $f, g$.