A high school social science teacher reads an article about a shopping mall that straddles the border between two municipalities with different minimum wages. The teacher could best use this example to support instruction on economics in which of the following?

a) Examining the relationship between income and expenditure.

b) Demonstrating the effects of economic policy on individuals.

c) Contrasting regional and national competition in retail markets.

d) Introducing the concepts of retail pricing systems and profit motives.