You are the project manager for the CSR Training Project, and $21,000$ customer service reps are invited to attend the training session. Attendance is optional. You have calculated the costs of the training facility, but the workbook expense depends on how many students register for the class. For every $5000$ workbooks created, the cost is reduced by a percentage of the original printing cost. The workbook expense is an example of which one of the following?

a$.$ Variable costs

b$.$ Fixed costs

c$.$ Indirect costs

d$.$ Parametric costs