Farmer: In the long run, it is counterproductive for farmers to use insecticides. Because insects' resistance to insecticides increases with insecticide use, farmers have to use greater and greater amounts of costly insecticides to control insect pests. Which one of the following most accurately describes the role played in the farmer's argument by the proposition that farmers have to use greater and greater amounts of costly insecticides to control insect pests? ["It identifies a phenomenon for which the argument's main conclusion offers a causal explanation.", "It is the argument's only conclusion.", "It is a claim for which a causal explanation is provided and which itself is used as direct support for the argument's only conclusion.", "It is the argument's main conclusion, but not its only conclusion."]