When there is less rainfall than normal, the water level of Australian rivers falls and the rivers flow more slowly. Because algae whose habitat is river water grow best in slow-moving water, the amount of algae per unit of water generally increases when there has been little rain. By contrast, however, following a period of extreme drought, algae levels are low even in very slow-moving river water. Which of the following, if true, does most to explain the contrast described above? ['Except during periods of extreme drought, algae levels tend to be higher in rivers in which the flow has been controlled by damming than in rivers that flow freely.', 'Australian rivers dry up completely for short intervals in periods of extreme drought.', 'During periods of extreme drought, the populations of some of the species that feed on algae tend to fall.', 'When algae populations reach very high levels, conditions within the river can become toxic for some of the other species that normally live there.']