Why the answer C is wrong? Isn't it necessary to assume "mental illness" and "physical disease" have such similarity. Nothing in the text mentions physical and suddenly talks about physical disease in its conclusion.
7. A researcher discovered that people who have low levelsof immune-system activity tend to score much lower on tests of mental healththan do people with normal or high immune-system activity. The researcher concludedfrom this experiment that the immune system protects againstmental illness as well as against physical disease.[c1] The researcher's conclusion depends on which of the followingassumptions? (Al[c2] High immune-systemactivity protects against mental illness better than normal immune system activitydoes. (8l[c3] Mental illnessis similar to physical disease in its effects on body systems. (C) People with high immune-system[c4] activitycannot develop mental illness. (Dl [c5] Mental illnessdoes not cause people's immune system activity to decrease. D (El [c6] Psychologicaltreatment of mental illness is not as effective as is medical treatment. [c1]因果型结论
because 'immune system' itself is basically working on physical health. This is a basic knowledge and hence this particular point is not emphasized.
That's why for this one focuses on the relationship between immune system and mental illness. D. rules out the possibility of reversed causal relationship and stands out to be the correct choice.