The following information was taken from the annual manufacturing overhead cost budget of Fergie Manufacturing.Variable manufacturing overhead costs$$92,400$Fixed manufacturing overhead costs$$55,440$Normal production level in labor hours$23,100$Normal production level in units$5,775$Standard labor hours per unit$4$During the year, $5,600$ units were produced, $18,340$ hours were worked, and the actual manufacturing overhead was $$151,200.$ Actual fixed manufacturing overhead costs equaled budgeted fixed manufacturing overhead costs. Overhead is applied on the basis of direct labor hours. Fergie's volume overhead variance is:a$.$$$1,680$ U$.$b$.$$$6,160$ U$.$c$.$$$7,840$ U$.$d$.$$$22,400$ U$.$