Please help with a detailed explanation, thanks very much!

Boulder Corporation uses a standard costing system. The following factory overhead and production data were reported in September: Standard fixed overhead allocation rate per direct labour $2 hour Estimated monthly direct labour hours 40,000 Standard direct labour hours for actual output in September 42,000 The fixed overhead production volume variance is: Select one: O A. $4,000 overapplied O B. $4,000 underapplied O C. $80,000 O D. $84,000