Which of the Following Is True of Perfectly Competitive Firms

Which of the following is true of perfectly competitive firms?

A) For a perfectly competitive firm, as long as the price derived from expanded output exceeds the marginal cost of that output, the expansion of output creates additional economic profits.
B) Producing at the profit-maximizing output level means that a firm is actually earning economic profits.
C) A competitive firm earning zero economic profit will be unable to continue in operation over time.
D) A perfectly competitive firm will operate in the short run only at price levels greater than or equal to average total costs.