Why do we consider the wacc to be both a marginal cost of capital as well as an average cost of capital?

The cost of the beginning work in process inventory was comprised of $3,000 of direct materials, $10,000 of direct labor, and $10,000 of factory overhead. costs incurred during the period were comprised of $15,000 of direct materials costs, and $100,000 of conversion costs. the equivalent units of production (eup) for the period were 9,000 for direct materials and 6,000 for conversion. the costs per eup were: