a concrete caisson is subjected to two separate loading conditions: a net uplift of $100$ kips or a net gravity load of $75$ kips. The loads are not applied simultaneously. The allowable soil skin friction is $200$ psf$,$ and the allowable soil bearing pressure is $4000$ psf$.$ neglecting the weight of the caisson, the required caisson diameter $($ft$)$ is most nearly:

$***$The answer is $($A$)$ but need the steps