The variable yearly income is a possible confounder in this study. Choose the best reason why. A variable is a confounder if its effect on the outcome cannot be distinguished from the effect on the outcome from different treatments. (In our context, the outcome is the monthly grocery bill, and the treatments are being a vegetarian and being a meat-eater.) There is already a noticeable difference between the two groups of participants in terms of dietary habits. It's possible the participants in the vegetarian group differ from the participants in the meat-eating group in other characteristics, such as yearly income, making it difficult to determine which variable is affecting the monthly grocery bill. A variable is a confounder if, based on its value, it prohibits some members of the sample from participating in the study. For this study, there may be some people who did not have a yearly income large enough to buy groceries monthly. So, it is possible that some of the people in the sample could not participate