GMAT Verbal Reasoning Practice Test 2

In a marketing study, consumers were given two unlabeled cartons of laundry detergent. One carton was bright green and yellow; the other was drab brown and gray. After using the detergent in the two cartons for one month, 83 percent of the consumers in the study reported that the detergent in the bright green and yellow carton cleaned better. This study shows that packaging has a significant impact on consumers’ judgment of the effectiveness of a laundry detergent.
Which of the following, if true, would most strengthen the conclusion drawn in the marketing study?