Mario loves to listen to music and is convinced that listening to music while taking a test helps him get higher test averages. To prove this, Mario conducts a study amongst his classmates in his AMDM class. With the permission of his teacher, he randomly selects 15 out of 30 students in his class to listen to music during their next test. The remaining 15 students are not allowed to listen to music while they take their test. He intends to compare the test scores from each group, hoping to prove his hypothesis that listening to music while taking a test increases test scores. What is the variable of interest in this study? O test grades the class O Mario O survey