Digital SAT Reading and Writing Practice Question #624

A recent ecological survey of the Yellowstone ecosystem demonstrated that the reintroduction of gray wolves not only regulated the elk population but also physically altered the geography of the region. As elk herds modified their grazing behavior to avoid topographically vulnerable valleys where wolves hunted, the suppressed willow and aspen stands along the riverbanks began to recover. This renewed vegetation stabilized the soil, significantly reducing riverbank erosion and altering the flow of the rivers themselves. However, ecologist Dr. Aris Thorne cautions against attributing the river transformations exclusively to the trophic cascade initiated by the wolves. He points out that _________. Which choice most logically completes the text?