Let $A = \{ 1,2,3 , \ldots , 10 \}$ and $f : A \rightarrow A$ be defined as
$$f ( k ) = \left\{ \begin{array} { c l }
k + 1 & \text { if } k \text { is odd } \\
k & \text { if } k \text { is even }
\end{array} \right.$$
Then the number of possible functions $g : A \rightarrow A$ such that $g o f = f$ is:\\
(1) ${ } ^ { 10 } \mathrm { C } _ { 5 }$\\
(2) $5 ^ { 5 }$\\
(3) 5 !\\
(4) $10 ^ { 5 }$