A researcher wants to test the effect of a new mindfulness app on anxiety levels. They randomly assign participants to two groups: one uses the app for 15 minutes daily, and the other does not. They measure anxiety levels before and after the two-week period. What is the independent variable in this study?
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A
The final anxiety levels of the participants.
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B
The two-week duration of the study.
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C
The use of the mindfulness app.
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D
The initial anxiety levels of the participants.