Assume that the cost formula for one of a company's mixed expenses is $10,000 + $4.00 per unit. The company's planned level of activity was 2,000 units and its actual level of activity was 2,200 units. The actual amount of this expense was $18,090. The spending variance for this expense is:

a. 1710 u

b. 1710 f

c. 310 u

d 310 f