Variable Costs Are Assumed to Be Linear, That Is, Variable

Variable costs are assumed to be linear, that is, variable costs vary directly in line with production of goods and delivery of services. Which of the following would result in costs not behaving in a strictly linear fashion?
Please select all that apply.

A) Bulk discounts for bulk purchases of raw materials.
B) Bonus payments to production employees.
C) Stepped fixed costs.
D) Overtime payments to production employees.