People who say that Dooney County is flat are clearly wrong. On flat land, soil erosion by water is not a problem. Consequently, farmers whose land is flat do not build terraces to prevent erosion. Yet I hear that the farms in Dooney County are dotted with terraces. The author's conclusion in the passage depends on the assumption that ['on flat land there is no soil erosion', 'the only terraces in Dooney County are on farmland', 'there are terraces on farmland in Dooney County which were built to prevent soil erosion', 'the only cause of soil erosion is water']