PrepTest 130, Section 1, Question 24
The Kiffer Forest Preserve, in the northernmost part(48 more words not shown)
Which one of the following,(6 more words not shown)
- AMost of the(15 more words not shown)
- BOnly some of(25 more words not shown)
- COnly some of(23 more words not shown)
- DThe bear population(18 more words not shown)
- EThe bear population(14 more words not shown)
Reading the argument
The preserve's bear population nearly doubled during eight years with the main road closed. Was closing the road responsible? Perhaps there was more food, fewer hunters, or more favorable weather during those years. The timing alone doesn't isolate the cause.
There's also the question of future growth. Even if closing the road helped, what if the area now holds as many bears as its food supply can support? Past growth wouldn't guarantee continued growth.
What we’re looking for
For the weakening question, I'd initially look for a different explanation of the increase, such as fewer hunters or more deer for the bears to eat. Let's see whether the choices challenge the forecast in that way or reveal another gap.
Working the answers
- A
Migration doesn't by itself separate the increase from the road closure. Bears might have moved in because the preserve became more attractive without the traffic. We also aren't told where they came from. Movement from outside the valley could increase its total population, while movement within the valley would not. A leaves that distinction open.
- B
This draws attention to a geographic distinction worth checking. Our evidence concerns the preserve, which occupies only part of the valley, while the conclusion predicts growth in the entire valley. Bears moving from another part of the valley into the preserve wouldn't add to the valley's total.
But B says only some of the increase came from that movement. Other increases could come from births or arrivals from outside the valley. B doesn't tell us what happened to the whole valley's population, so it doesn't remove the basis for the forecast as directly as E does.
- C
Some migration from outside the valley can add bears to the valley. The rest of the preserve's increase might come from movement within the valley or from births; the answer doesn't tell us. B and C aren't equivalent, and neither establishes the overall population trend. C's qualification that only some growth came from outside doesn't show that the valley failed to grow.
- D
A decline elsewhere in the valley could offset the preserve's increase. But how large was the decline? If it was smaller than the increase in the preserve, the valley could still have gained bears. D raises the right total-population issue without giving the net result.
- ECorrect
The whole valley's population stayed about the same during the eight years. That directly undermines using the preserve's near-doubling as evidence that continued closure will increase the valley's total. Local growth is compatible with no overall growth—for example, if bears shifted from other parts of the valley into the preserve.
E doesn't prove that every added preserve bear came from such a move, or that closing the road had no benefit. It shows that the overall increase the forecast relies on hasn't been established by the local increase. The argument changed from the preserve in its evidence to the valley in its conclusion.
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