The Chilton Corporation specializes in manufacturing one type of desk lamp. Chilton allocates variable manufacturing overhead costs on the basis of machine hours. Chilton budgeted .5 machine hours per lamp and allocates overhead at a rate of $1.80 per machine hour. Last year Chilton manufactured 23,000 lamps, used 13,800 machine hours and incurred actual overhead costs of $15,180. What was Chilton's variable manufacturing overhead efficiency variance last year? A. $9,660 favorable B. $4,140 unfavorable C. $4,140 favorable D. $9,660 unfavorable