A $40-$foot$-$diameter gravity thickener is used to thicken $80$ gallons per minute of primary sludge. The primary sludge is applied to the thickener at an initial sludge solids concentration of $4$ percent. Find the solids loading rate applied to the gravity thickener in pounds per square foot per day. Round to the nearest whole number.

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$26$ lb$/$ft$2/$d

$29$ lb$/$ft$2/$d

$22$ lb$/$ft$2/$d

$31$ lb$/$ft$2/$d