A drug that is highly effective in treating many types of infection can, at present, be obtained only from the bark of the ibora, a tree that is quite rare in the wild. It takes the bark of 5,000 trees to make one kilogram of the drug. It follows, therefore, that continued production of the drug must inevitably lead to the ibora’s extinction.
Which of the following, if true, most seriously weakens the argument above?
(A) The drug made from ibora bark is dispensed to doctors from a central authority.
(B) The drug made from ibora bark is expensive to produce.
(C) The leaves of the ibora are used in a number of medical products.
(D) The ibora can be propagated from cuttings and grown under cultivation.（E）
(E) The ibora generally grows in largely inaccessible places.
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.
16. 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 people’s immune-system activity to decrease.（D）
(E) Psychological treatment of mental illness is not as effective as is medical treatment.
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）
(E) high levels of stress first cause mental illness and then cause decreased immune-system activity in normal individuals | 
