Consider the Example of Cyr, a Researcher Who Studies the Role

Consider the example of Cyr, a researcher who studies the role of worked examples in engineering education. He measures students test performance and ratings of their instructors. He randomly assigns classes to condition. In some classes worked examples are shown, some where no worked examples are used, and a third set of classes in which students have access to worked examples if they elect to look at them.
-The independent variable in Cyr's research is

A) Engineering education
B) Worked example exposure
C) Test performance
D) Teacher ratings