Which of the following statements is true for a firm that uses variable costing?

<p><em><strong>a) The cost of a unit of product changes because of changes in the number of manufactured units.</strong></em></p> <p><em><strong>b) Profits fluctuate with sales.</strong></em></p> <p><em><strong>c) An idle facility variation is calculated.</strong></em></p> <p><em><strong>d) Product costs include "direct" (variable) administrative costs.</strong></em></p>