Brad reads a scatterplot that displays the relationship between the number of cars owned per household and the average number of citizens who have health insurance in neighborhoods across the country. The plot shows a strong positive correlation.

Brad recalls that correlation does not imply causation. In this example, Brad sees that increasing the number of cars per household would not cause members of his community to purchase health insurance.

Identify the lurking variable that is causing an increase in both the number of cars owned and the average number of citizens with health insurance.

• The number of different car brands
• Average health insurance costs in the United States
• Average annual salary per household
• The number of citizens in the United States who do not have health insurance