Most plants have developed chemical defenses against parasites. The average plant contains about 40 natural pesticides -- chemical compounds toxic to bacteria, fungi, and other parasites. Humans ingest these natural pesticides without harm every day. Ttherefore, the additional threat posed by synthetic pesticides sprayed on crop plants by humans is minimal. Each of the following, if true, weakens the argument EXCEPT: ['The synthetic pesticides sprayed on crop plants by humans usually have chemical structures similar to those of the natural pesticides produced by the plants.', 'Natural plant pesticides generally serve only as defenses against specific parasites, whereas synthetic pesticides are often harmful to a wide variety of organisms.', 'The concentrations of natural pesticides in plants are typically much lower than the concentrations of synthetic pesticides in sprayed crop plants.', 'Natural plant pesticides are typically less potent than synthetic pesticides, whose toxicity is highly concentrated.']