Variable costs are the costs that change when the quantity of ______________ changes, and they are ___________ when production is zero. The relationship between total variable costs (VC), cost per unit of output (v), and total quantity of output (Q) can be written as: VC = ___________________. Costs which do not change when the quantity of output changes during a particular time period are called________ . Total costs (TC) for a given level of output in a given period are defined as the sum of ______ and ________ : TC = _________________ = ______________________.