OG-48 请帮我解释句意

48. 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.

可否请大家帮我解释句意，感激！！

原文：免疫系统活动能力低的人精神健康差。所以所以免疫系统抵抗精神病和肉体的病。

D取非，是精神病引起免疫系统活动能力低，难以从该现象得出原文结论

感激lawyer_1百忙中帮我解惑，谢谢！！

标题错了，晕