Which of the Following Statements Is True of Variable Costs

Which of the following statements is true of variable costs incurred by firms?

A) These costs are independent of the number of goods produced by firms.
B) The rent and interest paid by firms are examples of variable costs.
C) These costs are the difference between what firms charge for their products and the fixed costs of the product.
D) These costs change directly with the units produced.
E) These costs refer to the total amount of margin made by a firm.