The Gray Company has a staff of 5 clerks in its general accounting department. 3 who work during the day perform sundry accounting tasks, the other 2 clerks who work in the evening are responsible for (1) collecting the cost data for the various jobs in process, (2) verifying manufacturing materials and labor reports and (3) supplying production reports to the supervisors by the next morning. The salaries of these 2 clerks who work at night should be classified as what type of cost?