The correct answer is (A).
Getting to the Answer: Review the method for the Experiment to identify which set of plants served as the control. The control group is the group that does not receive the experimental treatment and is used as a baseline to compare with the treated groups. According to the Experiment, "No herbicide was added to the soil in 10 pots. The other pots were...treated with 10, 20, 50, or 100 pm of either Herbicide A or B." Thus, the plants grown in soil without herbicide served as the control in the Experiment.
(A) Correct: this matches the analysis above; the set of plants grown in untreated soil was the control group
(B) Incorrect: since this set of plants received 10 ppm of Herbicide A treatment, it was not the control but a treated group
(C) Incorrect: since this set of plants received 10 ppm of Herbicide B treatment, it was not the control but a treated group
(D) Incorrect: since this set of plants received 100 ppm of Herbicide A treatment, it was not the control but a treated group
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