An effective power of a combination of \( 5 \) identical convex lenses which are kept in contact along the principal axis is \( 25 \, \text{D} \). Focal length of each of the convex lens is:
\( 20 \, \text{cm} \)
\( 50 \, \text{cm} \)
\( 500 \, \text{cm} \)
\( 25 \, \text{cm} \)