Unlike single persons, families today spend about the same percentage of their income on food as they did thirty years ago. The prices of nonfood items single persons purchase have risen faster than the price of food over the past thirty years. The amount of food eaten per capita today is identical to the amount of food eaten per capita thirty years ago. Single persons today, on average, purchase the same kinds of food items in the same quantities as they did thirty years ago.
Thirty years ago, the percentage of their income that single persons spent on food was twice what it is today. Given that incomes have risen over the past thirty years, we can conclude that incomes have risen at a greater rate than the price of food in that period.
Which one of the following, if assumed, helps most to justify the conclusion drawn above?