Alice reads a scatterplot that shows data for nine schools. It relates the percentage of students receiving free lunches to the percentage of students wearing a bicycle helmet. The plot shows a strong negative correlation. Alice recalls that correlation does not imply causation. In this example, Alice sees that increasing the percentage of free lunches would not cause children to use their bicycle helmets less. Identify the confounding variable that is causing Alice's observed association.

a. The number of bike helmets available
b. Parents' income
c. School funding
d. The number of free lunches available