Passage Summary: John is worried about getting bone cancer because a study found that male rats get it from drinking fluoride, so he thinks quitting fluoride is a guaranteed way to stay safe.

Conclusion: John believes he must stop drinking fluoridated water to ensure he does not develop bone cancer.

Reasoning: A study showed that fluoridated water caused bone cancer in 90 percent of male laboratory rats, though female rats were unaffected.

Analysis: John makes a massive leap from laboratory rats to humans, assuming that what happens to one species necessarily happens to another. He also treats fluoride as the sole cause of bone cancer, claiming that avoiding it will make him 'sure' to avoid the disease. Look for an answer that points out this jump in logic or the failure to consider other causes of cancer. It is a classic case of over-extrapolating from a limited study.