An activated sludge has a mean cell residence time $(c)$ of $10$ days. If the influent to the aeration tank is $220m\frac{g}{L}$BOD, the treatment target is $10m\frac{g}{L}$BOD, and the MLSS in the aeration tank is $6,000m\frac{g}{L}.$ The hydraulic residence time is most nearly? $Ks=60m\frac{g}{L},K_d=0\mathrm{.}1d^{-1},{}_m=3d,Y=0\mathrm{.}6$

a$.2\mathrm{.}0$ hr

b$.2\mathrm{.}5$ hr

c$.3\mathrm{.}0$ hr

d$.3\mathrm{.}5$ hr