In a study on commercial advertising, researchers divided participants into two groups, all of whom provided the same snacks. One group of participants watched a 10-minute popular comedy video first, and then watched 5 minutes about Commercials for food; another group of participants served as a reference group, and they watched a popular 15-minute comedy video. The study found that participants who watched commercials for food consumed significantly more snacks than the other group. The study believes that commercial food advertising can effectively increase people's consumption of snacks. If any of the following is true, it can weaken the inference of the above study? ? ['When subjects watched food-free ads, there was no significant difference between snack consumption and food ads.', 'When the researchers changed the popular comedy short film to tragedy short, the snack consumption of the group watching the food advertisement was still higher than that of the reference group.', 'The longer the food ad is played, the greater the group ’s snack consumption.', 'When researchers changed the popular comedy shorts to tragedy shorts, snack consumption in both groups dropped.']