Dr. Kang, a cognitive psychologist, conducts an experiment examining the effect of emotion on memory. He provides lists of 15 words to two groups of participants at his university. He puts the names of all the participants in a hat. The first 20 names he assigns to Group A and the last 20 he assigns to Group B. Group A is given a list of words that are very emotional in content (e. g., passion, murder). Group B is given a list of words that are neutral in content (e. g., houseplant, desk). He then measures how many words each group is able to remember after being distracted for 5 minutes by watching a video about the history of the university. He finds that Group A remembers 15% more words than Group B. Which of the following is the independent variable in Dr. Kang's study