Passage Summary: Workers at oil refineries who deal with a specific gas additive are getting sick, so the expert thinks everyone will start getting those same symptoms when that gas becomes common.

Conclusion: The general public will likely experience more headaches, fatigue, and shortness of breath once MBTE gasoline is widely used.

Reasoning: Workers who handle MBTE currently report a high number of these specific health complaints.

Analysis: The toxicologist is making a leap from a specific group (refinery workers) to the general population. To strengthen this, we need to assume that the exposure levels for the public will be sufficient to cause the same symptoms seen in the workers. Since this is a 'Strengthen EXCEPT' question, four choices will bolster this link—perhaps by ruling out other chemicals at the refinery—while the correct answer will either weaken the argument, be irrelevant, or provide no support at all.