An experiment was done by adding mass to an elastic band. What is the independent variable?

Is the independent variable the added mass or the stretch of the elastic band? In case you need it, here is the information:

Added mass (g): 5, 10, 15, 20, 25, 30

Stretch (cm): 1, 2, 3, 4, 5, 6An experiment was done by adding mass to an elastic band. What is the independent variable?
the added mass.