Only some strains of the tobacco plant are naturally resistant to tobacco mosaic virus, never becoming diseased even when infected. When resistant strains were experimentally infected with the virus, levels of naturally occurring salicylic acid in these plants increased fivefold; no such increase occurred in the nonresistant plants. In a second experiment, 50 nonresistant tobacco plants were exposed to tobacco mosaic virus, and 25 of them were injected with salicylic acid. None of these 25 plants showed signs of infection; however, the other 25 plants succumbed to the disease. Which one of the following conclusions is most strongly supported by the results of the experiments? ['Salicylic acid is not produced in strains of tobacco plants that are not resistant to tobacco mosaic virus.', 'It is possible to test an uninfected tobacco plant for resistance to tobacco mosaic virus by measuring the level of salicylic acid it contains.', 'Producing salicylic acid is at least part of the mechanism by which some tobacco plants naturally resist the disease caused by tobacco mosaic virus.', 'Tobacco plants that have become diseased by infection with tobacco mosaic virus can be cured by injecting them with salicylic acid.']