Ecologist: Landfills are generally designed to hold ten years' worth of waste. Some people maintain that as the number of active landfills consequently dwindles over the coming decade, there will inevitably be a crisis in landfill availability. However, their prediction obviously relies on the unlikely assumption that no new landfills will open as currently active ones close and is ttherefore unsound. The claim that there will be a crisis in landfill availability plays which one of the following roles in the ecologist's argument? ["It follows from the claim stated in the argument's first sentence.", 'It is the main conclusion of the argument.', 'It is a claim on which the argument as a whole is designed to cast doubt.', "It establishes the truth of the argument's conclusion."]