Answer:
A controlled experiment is a test where the person conducting the test only changes one (or very few) variable at a time in order to isolate the results and determine the effect or influence of the variable. Such an experiment may be one where all subjects involved in the experiment are treated exactly the same except for one deviation.
Explanation: If garlic repels fleas, then a dog that is given garlic every day will not get fleas. If sugar causes cavities, then people who eat a lot of candy may be more prone to cavities. If ultraviolet light can damage the eyes, then maybe this light can cause blindness.