AP Microeconomics Imperfect Competition — Worked Answer Explanations

Unit 4 · 12 questions explained

Below is a complete answer key for our AP Microeconomics Imperfect Competition practice questions. For each question you'll find the correct choice, a full written explanation of how to get there, and — for every wrong answer — a short note on exactly why it's tempting and where it goes wrong. Reading these straight through is one of the fastest ways to find the gaps in a unit before exam day.

Prefer to test yourself first? Take the timed Imperfect Competition practice test and come back here to review, or head back to the Imperfect Competition unit overview.

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Question 1 · Easy
Which of the following is a barrier to entry that enables a monopoly to earn long-run economic profit?
- A
  Economies of scale that allow existing firms to produce at lower cost than potential entrants.Correct
- B
  A large number of competing firms producing identical products.
  Why not B: Many firms with identical products describes perfect competition — the opposite of monopoly. Barriers to entry are low or absent in that market structure.
- C
  Perfect information for all consumers about available substitutes.
  Why not C: Perfect information aids competitive markets by helping consumers find alternatives. It does not create a barrier to entry that protects monopoly power.
- D
  Free entry by new firms attracted by positive economic profits.
  Why not D: Free entry eliminates long-run economic profit. Barriers to entry are what prevent new firms from entering and eroding monopoly profits.
Explanation
Barriers to entry protect a monopolist's long-run economic profit by preventing new competitors from entering the market. Common barriers include: (1) economies of scale (natural monopoly — large fixed costs mean one firm can serve the market more cheaply), (2) exclusive control of a key resource, (3) patents and copyrights, and (4) government licenses. Economies of scale can make it impossible for a small entrant to compete on price, effectively blocking entry.

Key takeaway
Barriers to entry (scale economies, resource control, patents, government licenses) allow monopolists to sustain long-run economic profit.
Question 2 · Easy
A monopolist faces the demand schedule below. What is the marginal revenue of the 3rd unit?

Quantity Price
1 $10∣∣2∣$ 8
3 $6∣∣4∣$ 4
- A
  $6
  Why not A: $6 is the price of the 3rd unit, not the marginal revenue. To sell one more unit, the monopolist must lower price on all units sold.
- B
  $2Correct
- C
  $18
  Why not C: $18 i s t o t a l r e v e n u e a t 3 u ni t s ($ 6 × 3). Marginal revenue is the change in total revenue from selling one additional unit.
- D
  $- 2$
  Why not D: MR is negative only in the inelastic range of demand. Here TR rises from $16 t o$ 18, making MR positive.
Explanation
Total revenue at 2 units: $8 \times 2 = \$ 16 $. T o t a l r e v e n u e a t 3 u ni t s :$ 6 \times 3 = $18$ . Marginal revenue of the 3rd unit: $MR_3 = \$ 18 - $16 = \$ 2 $. N o t e t ha tM R ($ $2$ ) is less than the price ( $\$ 6$) because the monopolist must lower the price on all units to sell one more. This price-MR gap is what gives the monopolist its market power.

Key takeaway
For a monopolist, $MR < P$ because selling one more unit requires lowering price on all units sold.
Question 3 · Easy
In which market structure do firms produce differentiated products, face a downward-sloping demand curve, and earn zero economic profit in the long run?
- A
  Perfect competition
  Why not A: Perfect competition features identical (homogeneous) products and a horizontal demand curve for each firm — not differentiated products.
- B
  Monopoly
  Why not B: Monopoly features a single seller who can sustain economic profit in the long run due to barriers to entry.
- C
  Monopolistic competitionCorrect
- D
  Oligopoly
  Why not D: Oligopoly has few interdependent sellers who may produce differentiated or identical products, but economic profit can persist in the long run due to barriers to entry.
Explanation
Monopolistic competition combines features of monopoly and perfect competition. Like monopoly, firms sell differentiated products and face downward-sloping demand curves — giving them limited pricing power. Like perfect competition, there are no significant barriers to entry; if firms earn positive economic profits, new firms enter with substitutes, shifting each firm's demand left until economic profit falls to zero. The long-run equilibrium has $P = A T C$ but $P > M C$ , creating excess capacity.

Key takeaway
Monopolistic competition: differentiated products + downward-sloping demand + free entry → zero long-run economic profit.
Question 4 · Easy
In game theory, a dominant strategy is best described as:
- A
  The strategy that maximizes a player's payoff only when the opponent cooperates.
  Why not A: A dominant strategy must be optimal regardless of what the opponent does — not just when the opponent cooperates.
- B
  The best strategy for a player regardless of what the other player does.Correct
- C
  The strategy that leads to the highest combined payoff for both players.
  Why not C: Dominant strategies maximize individual payoffs — they do not necessarily maximize the joint (combined) outcome. In a prisoner's dilemma, dominant strategies lead to an outcome inferior to the cooperative result.
- D
  A strategy adopted by the larger firm in an oligopoly to lead the market.
  Why not D: Market leadership (price leadership) is a different concept. A dominant strategy is a game-theoretic term about the best response regardless of rivals' choices.
Explanation
A dominant strategy is one that yields the highest payoff to a player no matter what strategy the opponent chooses. In the classic prisoner's dilemma, both players have 'confess' (or 'defect') as a dominant strategy, even though mutual cooperation would yield better combined outcomes. A Nash equilibrium occurs when all players are using their best responses to each other's strategies; if both players have dominant strategies, the Nash equilibrium is found by each player playing their dominant strategy.

Key takeaway
Dominant strategy = best choice regardless of opponent's action. Nash equilibrium = each player's best response to the other's strategy.
Question 5 · Medium
Compared to the competitive outcome, a profit-maximizing monopolist produces at a quantity where:
- A
  $P = M C$ , achieving allocative efficiency.
  Why not A: $P = M C$ is the competitive outcome. A monopolist sets $M R = M C$ , and since $M R < P$ , the monopoly price exceeds MC — allocative inefficiency.
- B
  Output is lower and price is higher, creating deadweight loss.Correct
- C
  Output is higher and price is lower because monopolists have economies of scale.
  Why not C: Monopolists restrict output to maximize profit. Higher output at lower price is closer to the competitive, not monopoly, outcome.
- D
  Price equals average total cost, ensuring zero economic profit.
  Why not D: $P = A T C$ (zero profit) is the long-run competitive equilibrium. Monopolists can sustain positive economic profits in the long run due to barriers to entry.
Explanation
A monopolist maximizes profit at $M R = M C$ , then charges the price consumers are willing to pay on the demand curve. Because $M R < P$ , the monopoly quantity is less than the competitive quantity ( $P = M C$ ), and the monopoly price is higher. The difference creates deadweight loss — units where consumer value exceeds marginal cost but are not produced. Monopolists are not productively efficient (do not produce at minimum ATC) nor allocatively efficient ( $P > M C$ ).

Key takeaway
Monopoly: $Q < Q^*$, $P > MC$, positive economic profit, and deadweight loss compared to competition.
Question 6 · Medium
In an oligopoly where firms collude to form a cartel, what is the most likely long-run outcome?
- A
  The cartel remains stable because all firms benefit equally from cooperation.
  Why not A: Each individual firm has an incentive to cheat (increase output above the agreed quota to earn more profit), making cartels inherently unstable over time.
- B
  The cartel breaks down as individual firms have incentives to cheat by producing above quota.Correct
- C
  The cartel converts the market into a perfectly competitive equilibrium over time.
  Why not C: Cartel breakdown leads to more competitive prices, but does not produce a perfectly competitive equilibrium unless all barriers to entry disappear.
- D
  Prices fall to marginal cost immediately because cartel agreements are illegal.
  Why not D: While cartels are illegal in most jurisdictions, legality does not automatically cause market prices to collapse to MC. Enforcement and market structure matter.
Explanation
A cartel acts like a monopolist: members agree to restrict output and charge a high price, earning collective profit above the competitive level. But each individual firm faces a prisoner's dilemma: if all others honor the quota, a single cheater can increase output and earn even higher profit (since the price is still above its MC). This dominant strategy — cheat — means cartels are inherently unstable. Historically, OPEC and other cartels have faced repeated internal breakdowns as members exceed production quotas.

Key takeaway
Cartels are unstable because each firm has a dominant strategy to cheat (produce above quota), undermining the agreed price.
Question 7 · Medium
A monopolist's deadweight loss is best represented graphically by:
- A
  The rectangle between the monopoly price and the competitive price, times the monopoly quantity.
  Why not A: That rectangle represents the transfer of consumer surplus to producer surplus (monopoly profit above the competitive price) — not the deadweight loss.
- B
  The triangle between the demand curve, the MC curve, and the monopoly quantity.Correct
- C
  The area below the MC curve and above the equilibrium price.
  Why not C: Producer surplus lies above the supply/MC curve and below price. The area below MC and above price has no standard welfare interpretation.
- D
  The entire area between the demand curve and the MC curve at all output levels.
  Why not D: That entire area represents total social surplus at the competitive output — not the deadweight loss, which is only the triangle between the monopoly and competitive quantities.
Explanation
At the competitive equilibrium, all units where demand (consumer value) exceeds MC are produced. A monopolist restricts output to $Q_{m} < Q_{c}$ . The units between $Q_{m}$ and $Q_{c}$ would have generated positive net social value (consumer value > MC) but are not produced. This foregone surplus is the deadweight loss — geometrically, the triangle bounded by the demand curve (top), the MC curve (bottom), and the vertical lines at $Q_{m}$ (left) and $Q_{c}$ (right).

Key takeaway
Monopoly DWL = triangle between demand, MC, and the gap from monopoly quantity to competitive quantity.
Question 8 · Medium
A natural monopoly exists when:
- A
  A single firm controls a key natural resource needed to produce the good.
  Why not A: Resource control is one type of monopoly barrier, but it is not the defining feature of a natural monopoly, which is specifically about cost structure.
- B
  A single firm can serve the entire market at a lower average cost than two or more firms could.Correct
- C
  Government grants an exclusive license to one firm to operate in a market.
  Why not C: A government-granted monopoly is a legal/regulatory monopoly, not a natural monopoly, which arises from the industry's cost structure — specifically large fixed costs and economies of scale.
- D
  A firm's average total cost is minimized at a quantity larger than market demand.
  Why not D: This describes a situation where the market cannot support even one efficient-scale producer — a related but distinct concept from natural monopoly.
Explanation
A natural monopoly arises when the production technology has large fixed costs and significant economies of scale throughout the relevant output range. This means the long-run average cost curve slopes downward across the entire market demand range. One firm can always serve the market at a lower per-unit cost than two firms sharing the market. Examples include electricity transmission, water distribution, and railroads. Regulators often allow natural monopolies to exist but regulate pricing (e.g., requiring $P = A T C$ for a fair-rate-of-return outcome).

Key takeaway
Natural monopoly: one firm serves the whole market at lower ATC than multiple firms — driven by large fixed costs and economies of scale.
Question 9 · Hard
A monopolist practicing perfect price discrimination (1st-degree price discrimination) compared to a single-price monopolist would:
- A
  Produce less output and earn less profit.
  Why not A: Perfect price discrimination allows the firm to capture all consumer surplus as profit and produce up to the competitive quantity — both profit and output are higher.
- B
  Produce the same output but charge different prices to different consumers.
  Why not B: A perfect price discriminator sells every unit at each buyer's maximum willingness to pay and expands output to where demand meets MC — output rises, not stays the same.
- C
  Eliminate deadweight loss by expanding output to the competitive level.Correct
- D
  Reduce producer surplus because charging different prices costs more to administer.
  Why not D: Administrative costs are not factored into this model. Perfect price discrimination increases producer surplus by capturing all consumer surplus.
Explanation
With perfect (1st-degree) price discrimination, the monopolist charges each consumer their maximum willingness to pay for every unit. This means the firm's MR curve becomes the demand curve itself (each unit earns its own price without requiring price reductions on earlier units). The firm produces all units where $W T P \geq M C$ — extending output to the competitive quantity. Deadweight loss is eliminated because all mutually beneficial transactions occur. However, all consumer surplus is captured as producer surplus — equity concerns remain.

Key takeaway
Perfect price discrimination: DWL = 0, output = competitive level, but all consumer surplus is captured by the producer.
Question 10 · Hard
A monopolistically competitive firm in long-run equilibrium operates with excess capacity because:
- A
  It faces a perfectly elastic demand curve and must accept the market price.
  Why not A: A perfectly elastic demand curve is characteristic of perfect competition, not monopolistic competition. Monopolistically competitive firms face downward-sloping demand.
- B
  It produces at a quantity less than the minimum efficient scale (minimum ATC).Correct
- C
  It produces where price equals marginal cost, fully exhausting potential gains from trade.
  Why not C: $P = M C$ is the allocative efficiency condition of perfect competition. Monopolistically competitive firms set $P > M C$ .
- D
  It is forced by government regulation to maintain a reserve production capacity.
  Why not D: Excess capacity arises naturally from the downward-sloping demand curve and free entry — no government mandate is required.
Explanation
In long-run equilibrium, free entry drives a monopolistically competitive firm's demand curve leftward until it is tangent to the ATC curve, giving zero economic profit ( $P = A T C$ ). But this tangency occurs on the downward-sloping portion of the ATC curve — to the left of minimum ATC. The firm produces less than the quantity that minimizes ATC, leaving unused productive capacity. This excess capacity is the price of product variety: consumers get differentiated goods, but average costs are higher than under perfect competition.

Key takeaway
Monopolistic competition in the long run: $P = ATC$ (zero profit), but $P > MC$ and output < minimum ATC — excess capacity exists.
Question 11 · Hard
Second-degree price discrimination (quantity discounts) differs from third-degree price discrimination primarily because:
- A
  Second-degree discrimination targets individual consumers; third-degree targets market segments.
  Why not A: This has the degrees reversed. First-degree discrimination targets individuals. Second-degree uses quantity blocks (self-selection). Third-degree separates identifiable groups.
- B
  Second-degree discrimination uses quantity blocks that allow consumers to self-select; third-degree separates consumers into identifiable groups.Correct
- C
  Third-degree discrimination requires lower prices in all markets, while second-degree requires higher prices.
  Why not C: Under third-degree discrimination, the firm charges higher prices to groups with less elastic demand and lower prices to those with more elastic demand — neither direction is universal.
- D
  Second-degree discrimination eliminates all deadweight loss; third-degree does not.
  Why not D: Only first-degree (perfect) price discrimination eliminates all deadweight loss. Second-degree and third-degree reduce but do not eliminate it.
Explanation
Price discrimination comes in three forms: (1st) Perfect — charge each consumer their exact willingness to pay. (2nd) Quantity-based — different per-unit prices for different purchase quantities (e.g., bulk discounts), allowing consumers to self-sort by how much they buy. (3rd) Market segmentation — identify distinct consumer groups with different price elasticities and charge each group a different price (e.g., student vs. adult tickets, airline peak vs. off-peak). Third-degree requires identifiable, separable groups — the firm prevents resale across segments.

Key takeaway
2nd-degree: quantity discounts/blocks (self-selection). 3rd-degree: separate identifiable market segments with different elasticities.
Question 12 · Hard
Two firms are deciding whether to advertise. The payoff matrix (Firm A profit, Firm B profit) is:

B Advertises B Doesn't Advertise
A Advertises (4, 4) (10, 2)
A Doesn't Advertise (2, 10) (8, 8)

What is the Nash equilibrium outcome?
- A
  Both firms advertise, each earning a profit of 4.Correct
- B
  Neither firm advertises, each earning a profit of 8.
  Why not B: If neither advertises (8, 8), either firm can unilaterally switch to advertising and raise its profit from 8 to 10. So 'neither advertises' is not a Nash equilibrium.
- C
  Only Firm A advertises, earning a profit of 10.
  Why not C: If only A advertises (10, 2), Firm B can deviate to advertising and raise its profit from 2 to 4. This is not a Nash equilibrium.
- D
  Firm A does not advertise; Firm B advertises — the Pareto-optimal outcome.
  Why not D: (2, 10) is not stable: Firm A would switch to advertising to raise its profit from 2 to 4. And 'Pareto-optimal' here is incorrectly applied — (8, 8) is actually Pareto superior to (4, 4).
Explanation
A Nash equilibrium is a set of strategies where no player can benefit by unilaterally changing their strategy. Check 'Both Advertise' (4, 4): if A deviates to not advertising, A gets 2 < 4 — no gain. If B deviates, B gets 2 < 4 — no gain. Nash equilibrium confirmed. 'Advertise' is a dominant strategy for each firm: A earns 4 (vs 2) if B advertises, and 10 (vs 8) if B doesn't — advertising always beats not advertising. This is a prisoner's dilemma: the dominant strategy equilibrium (4, 4) is worse than mutual cooperation (8, 8).

Key takeaway
Find Nash equilibrium by checking each cell for unilateral deviation incentives. Here advertising is a dominant strategy for both — classic prisoner's dilemma.

Quantity	Price
1	$10∣∣2∣$ 8
3	$6∣∣4∣$ 4

	B Advertises	B Doesn't Advertise
A Advertises	(4, 4)	(10, 2)
A Doesn't Advertise	(2, 10)	(8, 8)