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)

assumption题目有一部分可以把答案取非，如果能削弱就是答案。

LZ再想先