medium · GMAT Verbal

Which of the following, if true, most weakens the researchers' conclusion?

1. Many of the adults enrolled in the study reported that they already drink at least two full liters of water every single day as part of their ordinary daily routine.
2. Because participants knew they were in the water group, their self-reported energy gains likely reflect expectancy or placebo effects rather than a true physiological improvement.
3. The researchers running the study made no attempt whatsoever to track or to control the daily caffeine consumption of the participants in either of the two separate study groups involved.
4. The study itself lasted only a single month, whereas the genuine physiological benefits of consistently improved hydration may well take considerably longer than that to fully appear.
5. Several of the participants assigned to the water-drinking group reported that, throughout the study month, they needed to use the restroom far more frequently than usual.

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