the length of the pendulum the period of the pendulum the type of string used in making the pendulum the amount of mass used in making the pendulum
I gave my students this multiple choice question: Students were investigating the relationship between the length and period of a pendulum. They made a pendulum by using string to suspend a 0.5-kg mass from a fixed support. The students changed the length of the string and measured the time it took for the pendulum to complete ten cycles of motion. By dividing each time measurement by ten, they were able to calculate the period of the pendulum. What was the independent variable?

Only one answer is correct among these 4 choices:
- the length of the pendulum
- the period of the pendulum
- the type of string used in making the pendulum
- the amount of mass used in making the pendulum

Could you tell me which one is correct?

the length of the pendulum