Petrie Framing's cost formula for its supplies cost is $2,300 per month plus $6 per frame. For the month of March, the company planned for activity of 861 frames, but the actual level of activity was 856 frames. The actual supplies cost for the month was $7,790.

The activity variance for supplies cost in March would be:

a. $324 U.

b. $30 F.

c. $324 F.

d. S30 U.