An economics professor conducts an experiment to determine if viewing his online video reviews improve the performance of his students on exams. He uses only economics students enrolled in his classes. The treatment group will watch the videos prior to the first exam, and the control group will not watch the videos. Then the scores of the first exam will be compared. Which of the following variables’ effects could be mitigated by the random assignments? Check all that apply.

Intelligence
Study habits
Professor
Course textbook
Work schedule