A researcher wants to conduct an experiment to determine if eating a cookie before class each day improves student grades. He uses two psychology classes for the experiment, providing daily cookies to one and nothing to the other. At the end of the semester, the researcher compares the final grades of students in the two classes. What is the independent variable for this experiment?
a) Final grades of students
b) Type of psychology class
c) Eating a cookie before class
d) Semester duration