Which experimental finding would most challenge the assertion that zinc supplementation can shorten the duration of common colds?

• A. A bench experiment demonstrating that zinc binds to a viral protein in vitro but no in vivo human data on illness duration are provided.
• B. A case series describing gastrointestinal side effects of zinc supplements in people who told their physicians they took zinc during a cold.
• C. A randomized controlled trial showing no significant difference in cold duration between zinc-supplemented and placebo groups despite adequate dosing and proper blinding would challenge the assertion.
• D. An observational survey reporting that people who eat oysters once a year report fewer colds, without any controlled exposure or dosing information.

Answer: (C)