Water flows at a $2$ ft height in a rectangular open channel with a

base dimension of $4ft,$ a Manning roughness coefficient of $0\mathrm{.}02,$ and a

longitudinal slope of $0\mathrm{.}0005.$ What is most nearly the discharge in

cfs$?$

a$.21\mathrm{.}0$

b$.3360$

c$.9\mathrm{.}0$

d$.13\mathrm{.}3$