Highfill Corporation's variable overhead is applied on the basis of direct labor-hours. The standard cost card for product D80D specifies 6.3 direct labor-hours per unit of D80D. The standard variable overhead rate is $5.80 per direct labor-hour. During the most recent month, 1,000 units of product D80D were made and 6,400 direct labor-hours were worked. The actual variable overhead incurred was $40,920. Required: a. What was the variable overhead rate variance for the month? b. What was the variable overhead efficiency variance for the month? (Indicate the effect of each variance by selecting "F" for favorable, "U" for unfavorable, and "None" for no effect (i.e., zero variance). Input all amounts as positive values.)