Water flows in a $2\mathrm{.}4-$m$-$wide, rectanglular, finished concrete channel at a depth of $80cm.$ If the slope is $0\mathrm{.}002,$ the flow rate is nearest

$($A$)2\mathrm{.}2\frac{m^3}{s}$

$($B$)3\mathrm{.}4\frac{m^3}{s}$

$($C$)4\mathrm{.}6\frac{m^3}{s}$

$($D$)6\mathrm{.}2\frac{m^3}{s}$