Passage Summary: People who drank tea had faster-acting immune cells than those who drank coffee, so the tea must be the reason their immune systems were better.

Conclusion: The consumption of tea improved the immune system performance of the study participants.

Reasoning: Participants who drank only tea had immune cells that reacted twice as fast to germs compared to those who drank only coffee.

Analysis: The author is jumping from a correlation (tea drinkers have faster cells) to a causal conclusion (tea caused the speed). To make this stick, the author must assume there wasn't some other factor at play. Perhaps coffee actually slows down the immune system, or perhaps the tea drinkers already had healthier lifestyles. Look for an answer that rules out these alternative explanations or confirms that the two groups were essentially the same before the study began.