The wall shear stress in a fully developed flow portion of a $18-$in$.-$diameter pipe carrying water is $1\mathrm{.}85l\frac{b}{f}{t}^2.$ Determine the pressure gradient, del$\frac{p}{d}$elx, where $x$ is in the flow direction, if the pipe is $($a$)$ horizontal, $($b$)$ vertical with flow up$,$ or $($c$)$ vertical with flow down.

$($a$)$ del$\frac{p}{d}$elx$=$ lb$/$ft ${?}^3$

$($b$)$ del$\frac{p}{d}$elx$=l\frac{b}{f}{t}^3$

$($c$)$ del$\frac{p}{d}$elx$=l\frac{b}{f}{t}^3$