A researcher discovered that people who have low levels of immune-system activity tend to score much lower on tests 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 activity 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.
 D项心理疾病不会导致人的免疫系统下降到底和结论有什么关系呢？想不通呀，请大家指点哦，谢谢！

A researcher discovered that people who have low levels of immune-system activity tend to score much lower on tests of mental health than do people with normal or high immune-system activity.

研究只说明了low levels of immune-system activity与score much lower on tests of mental health的correlation,并没有说明究竟谁因谁果。

如你说言，D项的心理疾病不会导致人们免疫系统下降，排除掉了一种因果可能性，于是说明免疫系统下降会导致心理疾病，推出结论。

Just FYR

去掉Not ,d项说明精神疾病导致免疫功能下降，原文“the immune system protects against mental illness",因果倒置削弱，所以assumption正确.

那去掉C,People with high immune-system activity can develop mental illness,就是无因有果，不也削弱了吗？困惑！