Positive economic profits act as a signal that attracts new firms. Because there are no barriers to entry in perfect competition, new firms will enter the industry. This entry increases the market supply, which shifts the supply curve to the right and causes the market price to fall until economic profits are competed away and return to zero.

Zero economic profit means that total revenue equals total cost, where total cost includes all explicit (out-of-pocket) costs and all implicit (opportunity) costs. A normal profit is the minimum level of profit needed to keep a firm in business, and it is considered an implicit cost. Therefore, earning zero economic profit means the firm is earning a normal profit and is covering all its costs.

The positive profits attract new firms. As new firms enter, the market supply curve shifts to the right. In a constant-cost industry, input prices do not change as the industry expands. Therefore, supply will continue to increase until the price is driven back down to the original minimum average total cost, at which point economic profits are zero again.

In an increasing-cost industry, as the industry expands due to the entry of new firms, the demand for specialized inputs (like skilled labor or specific raw materials) increases. This increased demand drives up the price of those inputs, which in turn raises the cost curves (including the minimum ATC) for all firms in the industry. As a result, a higher market price is required to restore long-run equilibrium, leading to an upward-sloping long-run supply curve.

Productive efficiency means producing goods and services at the lowest possible cost per unit. In the long run, the process of entry and exit in a perfectly competitive market forces the price to the minimum point of the average total cost (ATC) curve. Firms that do not produce at this minimum cost point will incur losses and be driven out of the market.

Because all firms in a perfectly competitive market sell an identical product and there are many other sellers, a single firm has no market power. If it raises its price even slightly above the market price, rational consumers will simply buy the identical product from one of its many competitors at the lower market price. Consequently, the firm's sales will drop to zero.

The key skill here is understanding firm behavior in perfect competition, where firms decide output to maximize profit or minimize losses. A perfectly competitive firm is a price taker, meaning it accepts the market price as given, so its marginal revenue (MR) equals the price (P). To maximize profit, the firm produces the output level where marginal revenue equals marginal cost (MR = MC), or the largest Q where MR >= MC in discrete data. Profit or loss is determined by comparing P to average total cost (ATC) at that output, while shutdown occurs if P < average variable cost (AVC), as this means variable costs aren't covered. A common misconception is confusing short-run shutdown with long-run exit; shutdown means temporarily producing zero to avoid variable costs, while exit means permanently leaving the market. A useful strategy is to first calculate MC from changes in VC and find the Q where MR >= MC. Then, compare P to ATC for profit/loss and to AVC for shutdown decisions.

This question tests perfect competition firm behavior, specifically the shutdown decision in the short run. As a price taker at P = $12, the firm's marginal revenue equals $12 per unit. First, we find the profit-maximizing output by calculating marginal costs: MC from Q=2 to Q=3 is ($50-$42)/(3-2) = $8, and MC from Q=3 to Q=4 is ($65-$50)/(4-3) = $15. The firm produces where MR = MC, which occurs at Q = 3 (since $8 < $12 < $15). To determine shutdown, we need average variable cost (AVC) at Q = 3: with fixed cost of $25 (TC at Q=0), variable cost at Q=3 is $50-$25 = $25, so AVC = $25/3 = $8.33. Since P = $12 > AVC = $8.33, the firm should continue producing despite losses. A common misconception is that firms shut down whenever P < ATC; the correct rule is to shut down only when P < AVC. The transferable strategy is: find optimal output where MR = MC, calculate AVC at that output, and continue producing if P ≥ AVC.

The key skill here is understanding firm behavior in perfect competition, where firms decide output to maximize profit or minimize losses. A perfectly competitive firm is a price taker, meaning it accepts the market price as given, so its marginal revenue (MR) equals the price (P). To maximize profit, the firm produces the output level where marginal revenue equals marginal cost (MR = MC), or the largest Q where MR >= MC in discrete data. Profit or loss is determined by comparing P to average total cost (ATC) at that output, while shutdown occurs if P < average variable cost (AVC), as this means variable costs aren't covered. A common misconception is confusing short-run shutdown with long-run exit; shutdown means temporarily producing zero to avoid variable costs, while exit means permanently leaving the market. A useful strategy is to first calculate MC from changes in VC and find the Q where MR >= MC. Then, compare P to ATC for profit/loss and to AVC for shutdown decisions.

The market demand curve represents the relationship between the price and the total quantity demanded by all consumers in the market; it is downward sloping according to the law of demand. The demand curve for a single firm, however, is perfectly elastic (horizontal) at the market price because it is a price taker and can sell any quantity it chooses at that price.