Recently, the sales volume of a certain brand of laundry powder has increased significantly, and at the same time, the brand's expenditure on advertising has also increased significantly. The industry believes that the sales of this product are increasing due to the promotion of advertising. Which of the following, if true, would most weaken the conclusion? ["Few people noticed the brand's washing powder ads.", "Few buyers of the brand's laundry detergent pay attention to the brand's ads.", 'The advertising costs of this brand of detergent are not more than other brands of detergent.', "Consumers' Association received more complaints about this brand of detergent than other brands of detergent."]