The independent variable is the changing fan speeds.
Independent variable are changed or controlled in experiments.
I believe the answer is canon.
A round is a musical composition, a limited type of canon in which a minimum of three voices sing exactly the same melody at the unison but, with each voice beginning at different times.
I hope that this correct and that I helps :)❤️✨
That was the period when US shifted from its isolation/ non interference in global affairs policy
