Leaf beetles damage willow trees by stripping away their leaves, but a combination of parasites and predators generally keeps populations of these beetles in check. Researchers have found that severe air pollution results in reduced predator populations. The parasites, by contrast, are not adversely affected by pollution; nevertheless, the researchers' discovery probably does explain why leaf beetles cause particularly severe damage to willows in areas with severe air pollution, since __. Which of the following most logically completes the passage? ['where air pollution is not especially severe, predators have much more impact on leaf-beetle populations than parasites do', 'the damage caused by leaf beetles is usually not enough to kill a willow tree outright', 'the parasites that attack leaf beetles actually tend to be more prevalent in areas with severe air pollution than they are elsewhere', 'willows often grow in areas where air pollution is especially severe']