Quote:
A researcher discovered that people who have low levels of immune-system activity tend to score much lower on test of mental health than do people with normal or high immune-system activity. The researcher concluded from this experiment that the immune system protects against mental illness as well as against physical disease.
The researcher's conclusion depends on which of the following assumptions?
(A) High immune-system protects against mental illness better than normal immune-system activity does
(B) Mental illness is similar to physical disease in its effects on body systems
(C) People with high immune-system activity cannot develop mental illness.
(D) Mental illness does not cause people's immune-system activity to decrease
(E) Psychological treatment of mental illness is not as effective as is medical treatment.
B) and E) are irrelevant.
I think A) is wrong for the reason explained by soul_assassin. C) is also wrong for the same reason.
I also go with D).
The researcher is concluding that low immune system activity is causing mental illness, not other way around(mental illness causing low immune activity).
If assumption in D) is negated, the conclusion becomes invalid.