PrepTest 152, Section 4, Question 12
The Amazon River flows eastward into the Atlantic(74 more words not shown)
Which one of the following,(20 more words not shown)
- AIn many cases,(18 more words not shown)
- BMost of the(23 more words not shown)
- CMany species of(19 more words not shown)
- DThe Andes Mountains(13 more words not shown)
- EThere are very(17 more words not shown)
Reading the argument
How did descendants of Pacific fish end up on the other side of the mountains?
The Amazon flows toward the Atlantic now, and the Andes block the route west to the Pacific. But the ancestors of these fish lived in the Pacific, not the Atlantic. The scientists' proposed former connection makes sense as a possibility. What if the mountains weren't there when those fish lived? There might once have been a route through land that is now blocked.
What we’re looking for
I'd still want evidence before settling on that history. Could there have been some other route? A route through the Atlantic would have to fit the stated fact that the ancestors inhabited the Pacific but not the Atlantic. For the strengthener, I'd look for evidence of the earlier connection—perhaps that the mountains formed later than the relevant fish.
Working the answers
- A
The passage doesn't infer common ancestry merely from similar features. It states that the Amazon fish descend from these Pacific fish. A warning that similar-looking fish sometimes have different ancestors doesn't add support for the proposed river connection. We should use the ancestry given rather than replace it with an argument the passage didn't make.
- BCorrect
The fossil evidence places these Pacific fish before the Andes formed. That removes an apparent obstacle to the proposed earlier connection: the barrier we see today wasn't there during the relevant period. It fits the prediction that the mountains could be a later development. B doesn't prove the Amazon flowed west or that the fish actually followed that route, but it makes that history more plausible.
- C
Relationships between other Atlantic and Pacific fish don't show how these particular Pacific ancestors are connected with the Amazon. The answer doesn't establish an earlier westward course for the river. A broad relationship between the two oceans' fish leaves the specific historical route unresolved.
- D
The present extent of the Andes doesn't tell us when that barrier formed. Even a mountain range stretching the full length of the continent could have appeared after the proposed connection. We need evidence about the earlier situation, not more detail about the current obstruction.
- E
Being descended from saltwater fish isn't the same as the same individual fish needing to survive indefinitely in both environments. The question concerns the historical connection that could account for the ancestry. A statement that few species currently tolerate both environments doesn't provide evidence that the Amazon once flowed into the Pacific.
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