A researcher believes that motivation can backfire when tasks are difficult. 12 volunteers are asked to solve anagram puzzles. Each anagram has six letters; although the first 7 anagrams are easy, the last three are extremely difficult. The volunteers are randomly assigned to three groups. The first group is told that they will receive a $10 prize if they solve all ten anagrams, the second group is told they will receive a $50 prize, and the third group is told they will receive a 1$ prize (due to budgetary concerns, no prizes are actually awarded). At the end of the experiment, the first group solves 2.1 of the last 3 anagrams, compared to 0.8 for the second group and 1.1 for the third.
Which of the following labels would be the best choice to name the independent variable? Why is each of the other choices inferior?