Jan and Fran Hook are hairdressers with over 40 years of experience between them. Recently Obj. 4 they decided to combine their savings and buy a factory to produce men's hairpieces. Jan has assumed responsibility for administration, including the development of accounting procedures. Fran does not understand why the inventory value of a hairpiece should include plant and supervisory costs. How should Jan explain the process of placing a value on inventory to Fran?