12. Determine the length of the line segment

In the following figure, $ABCD$ is a rhombus. Diagonals $AC$ and $BD$ intersect at $O$. $E$ and $F$ are mid-points of $OA$ and $OB$ respectively. If $AC$ $=$ 36 $cm$ and $BD$ $=$ 48 $cm$, then $EF$ is:

 

 

The value of $EF$ $=$  $cm$