Passage Summary: A study showed Vitamin C helped acne, but then researchers realized many people knew what they were taking. When they looked only at the people who were kept in the dark, the benefit vanished, leading them to think the vitamin is useless.

Conclusion: Vitamin C likely provides no genuine benefit in reducing the severity of acne.

Reasoning: In a study where half the participants knew whether they were taking Vitamin C or a placebo, the benefit only appeared for those who knew; those who were unaware of what they were taking showed no difference in results.

Analysis: The argument concludes that Vitamin C is ineffective because the positive results were limited to the 'unblinded' group, suggesting a placebo effect. To weaken this, we need to find a reason why the 'blinded' group might have failed to show results even if the vitamin actually works. Perhaps the people who could tell what they were taking had more severe acne that responded differently, or maybe the knowledge of the treatment actually triggered a biological synergy. Look for an answer that suggests the two subgroups were not comparable or that the 'blinded' results are misleading.