Rachel randomly assigns her 48 4-year-old participants to either move together in sync to music for 3 minutes, or to move at different times. She examines whether this affects their level of social bonding. What is Rachel's independent variable?
A. Scores on the social bonding measure.
B. The amount of time they move (3 minutes)
C. Moving in sync or out of sync
D. The number of participants (48)