Passage Summary: A study found that eating a good breakfast makes people work better. They tested this by giving free breakfast to workers at one factory but not another, and only the breakfast group got more productive.

Conclusion: Providing workers with a healthy breakfast leads to an increase in their work productivity.

Reasoning: In a controlled comparison, the group that received the breakfast saw a rise in productivity while the group that did not receive it saw no change.

Analysis: This is a causal argument based on a study, so we want to rule out other factors that could have caused the difference. If Plant A and Plant B were different in other ways—like management style or equipment—the breakfast might not be the real cause. Look for an answer that ensures the only significant difference between the two plants was the breakfast. Strengthening a study often involves showing that the control group and the experimental group were truly comparable.