Mondo Snow Removal's cost formula for its vehicle operating cost is $1,060 per month plus $429 per snow-day. For the month of January, the company planned for activity of 11 snow-days, but the actual level of activity was 13 snow-days. The actual vehicle operating cost for the month was $6,310. The activity variance for vehicle operating cost in January would be closest to: a. $531 U b. $531 F c. $858 U d. $858 F