A researcher studies the rate of progress of a chemical reaction. The researcher fixes the temperature and pressure, varies the amount of catalyst used over a range of quantities, and observes the resulting reaction rate in each case. Which statement BEST describes why the investigation is an experimental study?

There is more than one independent variable to study.

The researcher manipulates at least one independent variable.

It is possible to get data with a range of values of an independent variable.

There is a cause-effect relation between the independent and dependent variables.