To test the effectiveness of an allergy drug, scientists begin with a pool of 12,000 allergy sufferers. one randomly selected group of 6,000 subjects is given the drug, and the other 6,000 are given a pill without the active ingredient. they are asked to report their allergy symptoms after 1 week, 1 month, and 3 months of treatment, and samples are also taken to measure the levels of histamines (allergy-related compounds) in the blood. what group is receiving the experimental treatment in this study?