What is the control variable in types of soft drink contains most fizz?

In an experiment to determine which type of soft drink contains the most fizz, the control variable would be the amount of time the soft drinks are allowed to sit before measuring the amount of fizz. This would ensure that all of the soft drinks are being tested under the same conditions and that any differences in the amount of fizz are due to the type of soft drink and not to other factors such as the amount of time they have been sitting.