Below are the choices:
The independent variable is the number of dry cells, and the dependent variable is the length of time the bulb works.
The independent variable is the length of time the bulb works, and the dependent variable is the number of dry cells.
The independent variable is the number of dry cells, and the dependent variable is the amount of energy available.
The independent variable is the amount of energy available, and the dependent variable is the number of dry cells.
I think the answer is The independent variable is the number of dry cells, and the dependent variable is the amount of energy available.