Arrange the following steps in the correct order to illustrate the impact of a decrease in consumer income on market demand in a perfectly competitive dairy market: A) Firms adjust output levels to maximize profit where marginal revenue equals marginal cost; B) The demand curve shifts leftward, leading to a decrease in equilibrium price; C) Consumers purchase less dairy due to reduced income, affecting overall market demand; D) The industry reaches a new equilibrium with lower prices and output levels.

When consumer income decreases, people have less money to spend, so they start buying less dairy. This change causes the demand curve for dairy to shift leftward, which is step C. As demand decreases, this leads to a lower equilibrium price in the market, which is step B. Firms notice they are selling less and adjust their production levels to maximize profit, where marginal revenue equals marginal cost, which is step A. Eventually, the industry reaches a new equilibrium with lower prices and output levels, completing the process in step D.