Longview Hospital performs blood tests in its laboratory. The following standards have been set for each blood test performed: Standard Price or Rate $2.75 per plate $15.00 per hour $7.00 per hour Standard Quantity or Hours 2.0 plates 0.2 hours 0.2 hours Direct materials. Direct labor Variable overhead... During May, the laboratory performed 1,500 blood tests. On May 1 there were no direct materials (plates) on hand; after a plate is used for a blood test it is discarded. Variable overhead is assigned to blood tests on the basis of standard direct labor-hours. The following events occurred during May: 3,600 plates were purchased for $9,540 . 3,200 plates were used for blood tests 340 actual direct labor-hours were worked at a cost of $5,550 The direct materials purchases variance is computed when the materials are purchased. 17. The variable overhead efficiency variance for May is A. $350 F B. $350 U C. $280 U D. $280 F