Answer: Years
Explanation:
An independent variable is the variable that is changed or controlled during an experiment, and this is done to test the effects on the dependent variable. So the dependent variable is affected by the independent variable and it is being tested and measured in a scientific experiment.
In the graph, time or years is the independent variable because it is not caused by the experiment. It just exists and it can cause a change in population (the dependent variable). As the years go by, the size of the population can increase or decrease, so it depends on the time elapsed.
