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, ttherefore, 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? ['The leaves of the ibora are used in a number of medical products.', 'The ibora generally grows in largely inaccessible places.', 'The ibora can be propagated from cuttings and grown under cultivation.', 'The drug made from ibora bark is expensive to produce.']