Following are costs for eight different items. Costs are shown for three different levels of units produced and sold. Classify each of the eight cost items as either variable, fixed, or mixed.

$\backslash$table$[[$Costs for Different Levels of Units Produced and Sold,At $$ units,At $2,000$ units,At $4,000$ units$],[$Direct labor,$$0,$$$13,800,$$$27,600$