Quisley's hypothesis is that listening to classical music improves intellectual performance. He randomly divides a mathematics class into two rooms. In room A, music by Mozart plays softly in the background during a lecture. In room B, the students listen 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.

ANSWER:
a