An experiment is designed to test whether the time a student spends studying is directly related to the grade a student earns on an exam. The students are divided into two groups – students who do not study for the test and students who do study for the test.
What modification should be made to improve the validity of the experiment?

a) The students should each have a different teacher.

b) The students should not be studying for the same exam.

c) The experiment is fine. No modifications are necessary.

d) The group of students who do study for the test should be subdivided into 3 groups – students who study 1 hour or less, students who study 1 – 2 hours, and students who study more than 2 hours.