Prolonged exposure to sulfur fumes permanently damages one' s sense of smell. In one important study, 100 workers from sulfur-emitting factories and a control group of 100 workers from other occupations were asked to identify a variety of chemically reproduced scents, including those of foods, spices, and flowers. On average, the factory workers successfully identified 10 percent of the scents compared to 50 percent for the control group. Each of the following, if true, weakens the argument EXCEPT: ['The chemicals used in the study closely but not perfectly reproduced the corresponding natural scents.', 'The subjects in the study were tested in the environments where they usually work.', 'Most members of the control group had participated in several earlier studies that involved the identification of scents.', 'Every sulfur-emitting factory with workers participating in the study also emits other noxious fumes.']