ANSWER:
She should use untreated sewage waste as the control group for this experiment.
Step-by-step explanation:
- In any experiment, a control group is the independent or isolated group that cannot affect the "outcome of an experiment".
- In this experiment, untreated sewage waste is the control group because untreated sewage wastage cannot affect the outcome of the experiment.
- The sewage waste is being treated with a chemical, bacteria X and bacteria Y all of which will have an impact on the result of the experiment.
- Only untreated sewage waste will not impact the outcome of the experiment.