The correct decreasing order of basic strength of the following species is.

${\mathrm{H}}_2\mathrm{O},{\mathrm{NH}}_3,{\mathrm{OH}}^{-},{\mathrm{NH}}_2^{}$

$1.<mspace\; width="1em"></mspace>{\mathrm{NH}}_2^{-}>{\mathrm{OH}}^{-}>{\mathrm{NH}}_3>{\mathrm{H}}_2\mathrm{O}$
$2.<mspace\; width="1em"></mspace>{\mathrm{OH}}^{-}>{\mathrm{NH}}_2^{-}>{\mathrm{H}}_2\mathrm{O}>{\mathrm{NH}}_3$
$3.<mspace\; width="1em"></mspace>{\mathrm{NH}}_3>{\mathrm{H}}_2\mathrm{O}>{\mathrm{NH}}_2^{-}>{\mathrm{OH}}^{-}$
$4.<mspace\; width="1em"></mspace>{\mathrm{H}}_2\mathrm{O}>{\mathrm{NH}}_3>{\mathrm{OH}}^{-}>{\mathrm{NH}}_2^{-}$