请教个问题

joycezuo1028

 看了答案还是不明白，请大家指点~

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 assumption?

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

e Psychological treatment of mental illness is not as effective as is medical treatment

the key is D

why?

yaoyao99

If: mentally ill (cause) => immune activity decreases (effect)

then you can't say that high immune activity (effect) => protects mental illness (cause)