If $f=\{(1,5),(2,6),(3,4)\}$ and $g=\{(4,7),(5,8),(6,9)\}$, then $gof$ = ?

Question

$\begin{array}{1 1} \{(4,7),(5,8),(6,9),(1,5),(2,6),(3,4)\} \\ \phi \\ \{(1,8),(2,9),(3,7)\} \\ none of these\end{array}$