A question on an economics exam asks: What happens in the market for margarine when income rises? Allison, an excellent student, shows the demand for margarine decreasing. Is she necessarily wrong? Why or why not?

If we consider Allison's answer wrong, we are implicitly assuming that margarine is a normal good and that the demand for margarine should decrease. But suppose Allison believes that margarine is an inferior good. In this case, her answer is correct. Good exam writers need to make explicit what they want students to assume.