PT130 · S1 · Q24 The Kiffer Forest Preserve

This may seem to weaken the author's causal assumption that the road closure is boosting the bear population, because it seems to provide an alternative explanation: the uptick isn't because of the road closure, it's because of migration. However, is that an alternative explanation or part of the same causal story? Author could say: having this road closed is creating a tranquil, undisturbed natural preserve, which attracts bears. Look, over the last 8 years, we have tons of new bears in this preserve, and (according to this answer) they didn't come from other parts of the valley, they migrated here from elsewhere! If we keep the road closed, we'll see more of that. As we'll see in correct answer E, if the new bears coming to the preserve were just coming from elsewhere in the valley, then they wouldn't be raising the total number of bears in the valley. It would be the same valley bear population as before, just re-shuffled into different areas. So being told that the new bears are from outside the valley strengthens the author's case. This answer doesn't clarify whether these bears have migrated from elsewhere in the valley or from outside the valley.

For all the reasons cited in the explanation for choice A, this also strengthens. By saying "only some new bears came from other parts of the valley", we learn that "some new bears came from outside the valley", which helps the author argue that the valley's bear population has increased and will continue to do so.

This has an identical impact to B, so neither answer could be correct. They're both using the formulation "only some", which reveals that "some of the increase came from outside the valley, some from inside the valley". The ones coming from outside the valley strengthen the author's case. The ones coming from inside the Valley (if they are outside the Preserve) would weaken the author's case.

This looks tempting: if the population in the preserve is going up, but in the area of the valley that's outside of the preserve it's going down, that looks like bears are traveling from other parts of the valley into the preserve. That scenario weakens the argument: it explains how the population in the preserve is climbing while allowing for the valley's bear population to have stayed the same overall. But is this answer actually saying that the bears have gone from the non-preserve parts of the valley to the preserve? No, we're just adding that. This answer ends up being less convincing than the best answer. Because this answer is at least suggestive of an alternate explanation that would hurt the author's argument (population increase in preserve is just from other parts of the valley), this could be a correct answer on a Weaken question. But it's not the best available answer in this case: E is way stronger at delivering the same objection.