A flat portion of an average total cost curve represents the various different levels of output at which the firm achieves: a) diseconomies of scale. b) constant returns to scale. c) decreasing returns to scale. d) economies of scale.

Because in the long term there are no more fixed inputs, the distinction between variable and fixed inputs disappears and there are no CFT or CVT curves. In reality, it is only necessary to look at the nature of the shape of the average cost curve in the long term. Suppose that technological constraints allow a company to choose between the construction of three plants of different sizes: small, medium and large.

This line is called the average long-term cost curve (CPLP) and shows the minimum unit cost for any production when all inputs are variable and it is possible to build all plant sizes. The dashed lines of the CPCP curves always correspond to higher costs for each production than can be obtained with plants of other sizes.

Obviously, the final choice will depend on market demand and consumer demand trends, generally favoring larger plants in future proposals. Otherwise, the medium plant will be the most attractive, due to its lower investment requirements. Usually the firm will have more than 3 sizes to choose from. When this number tends to infinity, the CPLP curve encloses the CP curves and is tangent to them.