问一道很奇怪的假定题

 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 conculded 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 assumption?

(B) Mental illness is similar to physical disease in its effects on body system

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

我能明白D是表示因果不能倒置，但是觉得从另一个角度来看选B也没有错，意思就是说Mental和physical没有差别，所以immune system既可以保护mental也可以保护physical，请问 B为什么不对呢？谢谢

而且选了D，physical disease就没用了。。。结论干吗还要提physical disease呢。。。

我说说我的理解：

文章researcher的结论是immune system能保护抵制mental illness和
        physical disease这两个疾病

简单的说就是immune system能起到作用，所以找researcher’s assumption??

将D取非: 若Mental illness有造成immune system下降，则WEAKEN结论，也就是若Ｄ成立则研究者的结论推不出：就是immune system能起到作用；

相对Ｂ选表示mental illness和physical disease相似为无关选项

请指教