A group of educators want to determine how effective tutoring is in raising students' grades in a math class, so they arrange free tutoring for those who want it$.$ Then they compare final exam grades for the group that took advantage of the tutoring and the group that did not. Suppose the group participating in the tutoring tended to receive higher grades on the exam. Does that show that the tutoring worked? If not, explain why not and suggest a confounding variable.