Novelli Corporation makes a product whose variable overhead standards are based on direct labor-hours. The quantity standard is 2.2 hours per unit. The variable overhead rate standard is $15.00 per hour. In September, the company produced 1,050 units using 2,300 direct labor-hours. The actual variable overhead rate was $17.10 per hour. The variable overhead efficiency variance for September is:

a. $171 U

b. $150 U

c. $171 F

d. $150 F