Let \((X, \mathscr{A}, \mu)\) and \((Y, \mathscr{B}, u)\) be two \(\sigma\)-finite measure spaces. Show that \(A \times N\), where \(A \in \mathscr{A}\) and \(N \in \mathscr{B}, u(N)=0\), is a \(\mu \times u\)-null set.