Shared Characteristic: Many Buyers

Steps:

• Recall perfect competition features many buyers and sellers with free entry and perfectly elastic firm demand.
• Recall monopoly features one seller with barriers to entry and downward-sloping firm demand.
• Compare options: Identify traits unique to each versus shared.
• Select the feature present in both: Numerous buyers on the demand side.

Why B is correct:

• Both structures assume a market with many buyers; perfect competition requires it explicitly, while monopoly faces market demand from multiple buyers per standard definition.

Why the others are wrong:

• A: Freedom of entry exists in perfect competition but not monopoly due to barriers.
• C: Many sellers define perfect competition but not monopoly, which has one seller.
• D: Perfectly elastic demand applies to firms in perfect competition but not monopoly's downward-sloping curve.

Final answer: B