Economies of Scale Will Create a Barrier to Entry in an Oligopoly

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 large fraction of total industry sales.
C) 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.
D) the industry's four-firm concentration ratio is less than 40 percent.