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.

17. The researcher’s conclusion would be most seriously weakened if it were true that

(A) there was a one-year delay between the completion of a pilot study for the experiment and the initiation of the experiment itself

(B) people’s levels of immune-system activity are not affected by their use of medications

(C) a few people with high immune-system activity had scores on the test of mental health that were similar to the scores of people who had normal immune-system activity

(D) people who have low immune-system activity tend to contract more viral infections than do people with normal or high immune-system activity

(E) high levels of stress first cause mental illness and then cause decreased immune-system activity in normal individuals

答案E，不明白？