Thomasson Air uses two measures of activity, flights and passengers, in the cost formulas in its budgets and performance reports. The cost formula for plane operating costs is $36,160 per month plus $2,038 per flight plus $1 per passenger. The company expected its activity in April to be 73 flights and 223 passengers, but the actual activity was 72 flights and 228 passengers. The actual cost for plane operating costs in April was $179,020. The activity variance for plane operating costs in April would be closest to: a. $6,137 U b. $6,137 F c. $2,033 U d. $2,033 F