How do controls improve an experiment?
Not only do controls establish a baseline that the results of an experiment can be compared to, they also allow researchers to correct for possible errors. If something goes wrong in the experiment, a scientist can check on the controls of the experiment to see if the error had to do with the controls.
In your case your anwser: B. by allowing for the experiment to test as many variables as possible
I hope this helped and Merry Christmas!