Should an experiment test only one variable at a time? Why or why not?

• A. Yes, an experiment should test only one variable at a time. That way, the experiment will have to be performed only once.
• B. No, it is not necessary to test only one variable per experiment, especially when time is of the essence.
• C. As long as the experiment is repeated a sufficient number of times, it does not matter how many variables are used.
• D. Yes, an experiment should test only one variable at a time. This ensures that the experimental outcome is clearly due to one identifiable factor.