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A manufacturing firm produces multiple families of products requiring various combinations of different types of parts. The manufacturer has identified various cost pools, one of which consists of materials handling costs. This cost pool includes the wages and employee benefits of the workers involved in receiving materials, inspecting materials, storing materials in inventory, and moving materials to the workstations; depreciation and maintenance of materials handling equipment (e.g., forklift trucks); and costs of supplies used as well as other related costs. Of the following, the most appropriate cost driver for assigning materials handling costs to the various products most likely is