If variable cost rises from $60 to $100 as output increases from 15 to 20 units, the marginal cost of the twentieth unit

Advances in technology and growth in human capital ________ because ________

A) shift the productivity curve downward; labor and capital become less productive B) shift the productivity curve upward; labor and capital become less productive C) shift the productivity curve upward; labor and capital become more productive D) do not shift the productivity curve; there is a movement along the productivity curve E) shift the productivity curve downward; labor and capital become more productive

Economies of scale will create a barrier to entry in an oligopoly industry when

A) a firm's minimum efficient scale occurs where long-run average total costs are constant. B) the typical firm's long-run average total cost curve reaches a minimum at a level of output that is a small fraction of total industry sales. C) the industry's four-firm concentration ratio is less than 40 percent. D) the typical firm's long-run average total cost curve reaches a minimum at a level of output that is a large fraction of total industry sales.