The independent variable is the number of dry cells, and the dependent variable is the length of time the bulb works.
Explanation:
From this experiment, we can clearly ascertain that the dependent variable is the length of time the bulb works and the independent variable is the number of dry cells.
The independent variable is usually the cause of the phenomenon observed. The dependent variable is the effect produced by the independent variables. The number of battery is independent. Duration of time the bulb works depends on the number of battery used.