A certain experimental fungicide causes no harm to garden plants, though only if it is diluted at least to ten parts water to one part fungicide. Moreover, this fungicide is known to be so effective against powdery mildew that it has the capacity to eliminate it completely from rose plants. Thus this fungicide, as long as it is sufficiently diluted, provides a means of eliminating powdery mildew from rose plants that involves no risk of harming the plants. Which one of the following is an assumption on which the argument depends? ['The effectiveness of the fungicide does not depend on its being more concentrated than one part in ten parts of water.', 'When the fungicide is sufficiently diluted, it does not present any risk of harm to people, animals, or beneficial insects.', 'If a fungicide is to be effective against powdery mildew on rose plants, it must eliminate the powdery mildew completely.', 'Powdery mildew is the only fungal infection that affects rose plants.']