In Some Industries Where Firms Experience Declining Average Total Costs

In some industries where firms experience declining average total costs over the full range of output that consumers are willing to buy,

A) a smaller firm will always have lower per-unit costs.
B) many firms will tend to emerge from the competitive process.
C) a single large firm will develop, and it will have cost advantages that protect it from potential rivals.
D) a single large firm will develop, and it will buy out any smaller rival firms to avoid the small firms' production at a lower per-unit cost.