Final answer:
In the described experiment, the change in diameter of softball bats is the independent variable, while the distance the softball travels is the dependent variable.
Step-by-step explanation:
In an experiment measuring the effect of different diameters of softball bats on how far the softball travels, the change in diameter is called the independent variable.
The independent variable is the one that is manipulated or controlled by the person conducting the experiment. On the other hand, the dependent variable is the one that is affected by the change, which in this case would likely be the distance the softball travels. Control variables are factors that are kept constant to prevent them from influencing the outcome of the experiment.