48. 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 poeple with normal or high immune-system activity. The researcher concluded form thsi 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 system. (C)People with high immune-system activity cannot develop mental illness. (D)Mental illness does not cause poeple's immune-system activity to decrease. (E)Psychological treatment of mental illness is not as effective as is medical treatment. The answer is D.

我觉得B也是无法排除的，搜了前面的帖子也没有答案，这里mental illness跟physical disease明显不一样

哪位牛牛来解释一下，谢谢