Bill and Betty are running a study looking at the impact of TV violence in children's responses to being scared. In the experimental session, Bill shows children clips from the cartoon "The Incredibles" where the bad guys are trying to kill the superhero children. In the control condition, Betty shows children clips from the cartoon Frozen where the snowman Olaf is singing. The results show that children who saw The Incredibles clip rated themselves as being more scared than children who saw the Frozen clip.
What is the independent variable?
a. Running a study
b. Number of children
c. Children ratings of being scared
d. film clip (The Incredibles' scary clip v. Frozen's singing clip)