A uniform chain of length `l` and mass `m` is lying on a smooth table and one-third of its length is hanging vertically down over the edge of the table. If `g` is acceleration due to gravity, work required to pull the hanging part on to the table is
Options:
(a) `mgl`
(b) `(mgl)/3`
(c) `(mgl)/9`
(d) `(mgl)/18`