15.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 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 activtiy tend to contract more viral infections than do people with normal or high immune-system activitiy.

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

　　16.A researcher studying drug addicts found that， on average， they tend to manipulate other people a great deal more than nonaddicts do. The researcher concluded that people who frequently manipulate other people are likely to become addicts.

　　Which of the following， if true， most seriously weakens the researcher's conclusion？

　　（A） After becoming addicted to drugs， drug addicts learn to manipulate other people as a way of obtaining drugs.

　　（B） When they are imprisoned， drug addicts often use their ability to manipulate other people to obtain better living conditions.

　　（C） Some nonaddicts manipulate other people more than some addicts do.

　　（D） People who are likely to become addicts exhibit unusual behavior patterrns other than frequent manipulation of other people.