请教NN一道逻辑, thank you!

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.
            (D)

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

B其实是无关选项啦，body system扩大了范围，这个body system是题目中没有提到的。

读题干要知道文章的FOCUS，很明显，本题关键在IS activity, mental illness

因此假设就是要找他们之间的关系。

选项中有提到两项的是A C D

仔细看A的重点其实是high i-s and normal i-s

b，并没有明确表明他们之间的因果关系，或是一种逻辑联系；

然而D确实在通过否定另一种可能性之后确定了他们之间的逻辑关系。这种可能性是可能存在的，而削弱结论的。