og cr 7th question

7.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 erperiment that immune system protects against mental illness as well as against physical disease.

The researcher's conclusion depends on which of the following assumptions?

correct answer given is d.

(d)mental illness does not cause people's immune system activity to decrease.

不知道这题讨论过没有，我反正是懵了。望指教！

这是OG补充材料(紫色)的第7题....