Quisley's Hypothesis Is That Listening to Classical Music Improves Intellectual

Quisley's hypothesis is that listening to classical music improves intellectual performance. He randomly divided a mathematics class into two rooms. In room A, music by Mozart played softly in the background during a lecture. In room B, the students listened to a lecture without background music. The independent variable is

A) whether music played during a lecture.
B) mathematical performance.
C) the level of mathematics taught.
D) present only in the control group.