Question

Explain why it is important to use only one independent variable at a time in a given investigation.

Answer 1

The reason you only use one independent variable is because you wouldn't know what to attribute changes to in your experiment if multiple variables were able to change.

The idea of an independent variable to figure out how it changes by doing something. If you have more than one of these variables, then you have more than one thing changing and you will have no idea what change is associated with which variable.