A study published in 2007 by Christopher Johnson, professor of music education and music therapy at the University of Kansas, revealed that students in elementary schools who took private music lessons scored around 20 percent higher in math scores on standardized tests, compared to students who did not take private music lessons.

Which of the following could be a potential confounding variable?

Whether the parents hired a math tutor

Socioeconomic status

Household income

All of them.

(Can choose multiple answers)