please help with a detailed explanation, Thanks very much!

Hogle Mfg. Co. uses a standard costing system. The standard time to produce one unit is 4 hours, and normal production is 3,000 units monthly. Overhead costs were estimated to be $135,000. The standard variable overhead rate is $5 per machine hour. During April the following results were recorded: Units produced 3,100 Units sold 2,800 Machine hours required 12,800 Actual overhead costs $136,000 The fixed overhead production volume variance was: Select one: O A. $1,000 U O B. $2,500 F O C. $1,500 F O D. $5,000 U