Passage Summary: Violent prisoners usually eat poorly. When researchers fed some of them healthy food, they became less violent, which supposedly proves that bad food causes violence.

Conclusion: The experimental results confirm that poor nutrition is a cause of violent behavior.

Reasoning: Violent inmates were observed choosing low-nutrient foods, and when a group of them was switched to a high-nutrient diet, their behavior improved over four months.

Analysis: The argument moves from a correlation and a single-group experiment to a definitive causal conclusion. To strengthen this, we need to eliminate potential 'lurking variables' or alternative explanations. For instance, did the behavior of the inmates who *weren't* given the new diet stay the same? If everyone's behavior improved regardless of diet, the experiment proves nothing. Look for an answer that provides a control group or suggests that the improvement wasn't caused by something else, like a change in prison leadership or seasonal shifts in behavior.