Answer:
A. true (unless directly referencing the independent variable)
Step-by-step explanation:
A fair test usually has a dependent variable, an independent variable, and a controlled variable. Usually you change the independent variable and see how the dependent variable reacts. Controlled variables are variables that don't get changed at all.
If you want to do a fair test, then you usually only change one thing at a time, which would then be the independent variable. If you had multiple independent variables (things that you change), the results might be warped and you couldn't clearly see how changing something affected the dependent variable.