I need detailed help with steps as to how to solve this problem, and why answer whichw was C) is what it is :

Welnor Corporation's flexible budget cost formula for supplies, one of the variable overhead costs, is $2.61 per unit of output. The company's flexible budget performance report for last month showed a $6,840 unfavorable spending variance for supplies. During that month, 17,100 units were produced. Budgeted activity for the month had been 16,700 units. The actual cost per unit for indirect materials must have been closest to :

A) $3.41 B) $2.61 C) $3.01 D) $3.49