Dubberly Corporation's cost formula for its manufacturing overhead is $30,600 per month plus $64 per machine-hour. For the month of March, the company planned for activity of 7,900 machine-hours, but the actual level of activity was 7,880 machine-hours. The actual manufacturing overhead for the month was $558,610. The activity variance for manufacturing overhead in March would be closest to: Select one: O a. $22,410 U O b. $1,280 U O c. $1,280 F O d. $22,410 F