During the winter of 1973-74, a general system of wage and price controls (including a price ceiling on gasoline) was in force in the United States. At the beginning of 1974, some oil-producing countries imposed an oil embargo (a legal prohibition on commerce) on the West. In the spring of 1974, price controls were abolished. Before the oil embargo, the price ceiling on gasoline had no noticeable effect on the market. What is the most likely explanation for this?