A researcher wants to investigate if a new type of cognitive therapy improves symptoms of depression. One group of patients receives the new therapy, while another group receives the standard therapy. The researcher measures depression levels before and after the eight-week treatment period. In this study, what is the independent variable?
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A
The patients' depression levels at the end of the study.
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B
The type of therapy received (new vs. standard).
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C
The duration of the treatment period (eight weeks).
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D
The improvement in depression symptoms.