Problem $19$: A proposed storm sewer will have a slope of $\backslash (0\mathrm{.}20\backslash \%\backslash ).$ The design flow for the line has been determined to be $16$ cfs$.$ The Manning's roughness coefficient of $0\mathrm{.}012$ is constant for all depths of flow. The minimum circular pipe size $($in$.)$ that will accommodate the design flow is most nearly:

$($A$)30$

$($B$)24$

$($C$)18$

$($D$)12$