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.

16. 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 system.

(C) People with high immune-system activity cannot develop mental illness.

(D) Mental illness does not cause people’s immune-system activity to decrease.（D）

(E) Psychological treatment of mental illness is not as effective as is medical treatment.

怎么答案是D呢，我认为根本和原文没有关系呀。而B，恰恰是把原文的结论和experiment加强了联系，虽然不是很标准的assumption但是4个选项中关联度最高的呀？

请NN们帮我解答一下。（链接和搜索都找不到有人讨论过）