To study the effects of listening to music while doing math, a researcher plays classical music while one group of participants completes math problems and music with a fast tempo while another group of participants completes math problems. After the experiment is over, the researcher realizes that the volume for the classical music was much lower than the volume for the other music. In this example, the volume of the music is a(n) ___ variable.
1) dependent
2) confounding
3) independent
4) control