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 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.

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

Reasoning-system activity can inhibit mental illness as it does physical illness．While it is possible that mental i11ness might itself depress immune-system activity, the researcher assumes that this is not the case．If mental illness caused a decline in immune-system activity, then lower levels of immune-system activity would be expected as a result，and the higher levels would merely indicate the absence of illness rather than any effect on illness.

答案选D。我选C，有点糊涂，请大牛指点一二。谢谢