Clinton's Engine Company manufactures part AT168 used in several of its engine models. Monthly unit production costs for 1,000 units are as follows: Direct materials $ 40 Direct labor 20 Variable overhead costs 30 Fixed overhead costs 10 Total $100 It is estimated that 10% of the fixed overhead costs assigned to AT168 will no longer be incurred if the company purchases AT168 from the outside supplier. Clinton's Engine Company has the option of purchasing the part from an outside supplier at $85 per unit. If Clinton's Engine Company purchases 1,000 AT168 parts from the outside supplier per month then its monthly operating income will increase by $6,000 O decrease by $3,000 increase by $5,000 decrease by $5,000