A falling$-$head test is to be performed on a soil sample whose hydraulic conductivity is estimated to be $2\mathrm{.}5105$ m$/$s$.$ The sample$$s cross$-$sectional area is $12\mathrm{.}5$ cm$2$ and its length is $7\mathrm{.}5$ cm$.$ What diameter standpipe should you use if you want the head to fall from $31$ cm to $19\mathrm{.}5$ cm in $6$ minutes? Assume water temperature $=20$C$.$