The incidence of serious animal-induced allergies among current zoo employees is lower than that among the general population. Among members of the general population who have spent as much time with animals as zoo employees typically have, the percentage with animal-induced allergies is significantly more than 30 percent. Zoo employees tend to develop animal-induced allergies that are more serious than those of other people who spend equally large amounts of time with animals. There is no occupation for which the risk of developing an animal-induced allergy is higher than 30 percent.
People who have spent a lot of time in contact with animals often develop animal-induced allergies, a significant percentage of which are quite serious. In a survey of current employees in major zoos, about 30 percent had animal-induced allergies. However, a zoo employee who develops a serious animal-induced allergy is very likely to switch to some other occupation.
Which of the following hypotheses receives the strongest support from the information given?