Clark Company manufactures a product with a standard direct labor cost of 2 hours at $18.00 per hour. During July, 2,000 units were produced using 4,200 hours at $18.30 per hour. The labor efficiency variance was Select one: O a. $3,600 F O b. $3,600 U O O c. $3,660 U d. $3,660 F. e. $3,680 U O