A 6. All other changeable factors that must
be kept the same to ensure a fair test
(what you keep the same).

a constant variable?

Explanation:

A constant variable is any aspect of an experiment that a researcher intentionally keeps unchanged throughout an experiment.

Experiments are always testing for measurable change, which is the dependent variable. You can also think of a dependent variable as the result obtained from an experiment. It is dependent on the change that occurs