Robust crops not only withstand insect attacks more successfully than other crops, they are also less likely to be attacked in the first place, since insects tend to feed on weaker plants. Killing insects with pesticides does not address the underlying problem of inherent vulnerability to damage caused by insect attacks. Thus, a better way to reduce the vulnerability of agricultural crops to insect pest damage is to grow those crops in good soil -- soil with adequate nutrients, organic matter, and microbial activity. Which one of the following is an assumption on which the argument depends? ['The application of pesticides to weak crops fails to reduce the extent to which they are damaged by insect pests.', 'Growing crops without the use of pesticides generally produces less robust plants than when pesticides are used.', 'Crops that are grown in good soil tend to be more robust than other crops.', "The application of nutrients and organic matter to farmland improves the soil's microbial activity."]