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C is corrent.  The requirement is to calculate the variable overhead efficiency variance.  C is corrent because the variable overhead efficiency variance is calculated by deducting the standard labor hours from the actual labor hours and multiplying by the standard variable overhead rate.  In this case the variable overhead efficiency variance is equal to $8,000 favorable [(11,000 – 12,000) × $8.00].
A is incorrect. the overhead efficiency is equal to the difference between standard hours and actual hours multiplied by the standard variable overhead rate. 
B is incorrect. The overhead efficiency is equal to the difference between standard hours and actual hours multiplied by the standard variable overhead rate. 
D is incorrect. The overhead efficiency is equal to the difference between standard hours and actual hours multiplied by the standard variable overhead rate.