Determine the length of a rectangular clarifier for a flow of $3$ mgd if the design detention time is $2$ hours, the tank has a depth of $8$ ft$,$ and the L : W $=4$:$1.($A$)130$ ft$($B$)120$ ft $($C$)110$ ft $($D$)100$ ft$4.$ Determine the minimum number of rapid sand filters necessary to treat a flow of $20$ mg