B is the correct answer. That's why it's important to have only ONE variable in a controlled experiment. Natural or unnecessary factors can affect your findings.
C) All variables are measured to provide data for the experimental results.
A is untrue because the independent variable will be changed to see how it changes the results.
B is untrue because the controlled variable will not be affecting the experiment because it is unchanging throughout the experiment.
D is untrue because the controlled variable will NOT be changing throughout the experiments.