Supply and Demand Drive Price Changes

Steps:

• Identify the core mechanism of price determination in a free market: interaction of supply and demand.
• Recall that prices adjust to balance quantity supplied and demanded.
• Evaluate each option against this principle, focusing on direct influences.
• Select the option that directly affects the supply-demand equilibrium.

Why A is correct:

• The law of supply and demand states that price changes occur when shifts in supply or demand curves alter equilibrium, as price rises with increased demand or falls with increased supply.

Why the others are wrong:

• B: Externalities like costs or benefits are market failures, not primary drivers in free markets; they require intervention.
• C: Government decisions involve regulation or intervention, absent in free markets.
• D: Public goods are non-excludable and not priced via markets, so their quantity changes don't affect private product prices.

Final answer: A