As part of her annual review of her company’s budgets versus actuals, Mary Gerard isolates unfavorable variances with the hope of getting a better understanding of what caused them and how to avoid them next year. The variable overhead efficiency variance (with direct labor-hours as the cost-allocation base) was the most unfavorable over the previous year, which Gerard will specifically be able to trace to

a. Actual variable overhead costs below allocated variable overhead costs.
b. Actual production units below budgeted production units.
c. Standard direct labor-hours below actual direct labor-hours.
d. The standard variable overhead rate below the actual variable overhead rate.