The correct answer is letter A. The disk soaked in distilled water is the controlled variable because it is the only differential factor from all of the other variables which are all disinfectants. Controlled variables are elements that are not changed throughout the experiment. When something is unchanged it is better for scientists to analyze the relationships of other variables better.
I need to know the possible answers.
Depends on the situation Usually the hardest would be blood samples of the Unsub. Again It depends on the situation.
May have once had liquid water, has a reddish color from iron, is a terrestrial planet,
