The definition of a control variable in an experiment is a part of it that is experimented, but it is purposefully kept the same throughout. If it is (C), why can't it be the others? Well, it cannot be (A) because it the dependent variable is the thing that is being measured in the experiment, so it will change over time, and not going to be kept the same. Lastly, it cannot be (B) theories because theories are always changing. Theories, due to new scientific knowledge, is going to be different as scientists learn new things. Therefore, (C), controls would be your answer. Hope this helps!
