Self-study
The Amazon River flows eastward into the Atlantic Ocean from its source in the western part of South America. ███ ████ ███████ █████ ███ ██████ █████ ██ ███ ███ ███ ████ ███ ███████ █████ ██ ███ ████ ██ ███ █████ ██████████ ███ ███████ ██████████ ████ ████ ███████ ███ ██████ ███ █████████ ████ ███████████ █████████ ████ █████ ██ ████ █████████ ███ ███████ █████ ███ ███ ███ █████████ ███ ████ ███████ ████ ██████████ ███████████ ████ ███ ██████ █████ ████ ██████ ████ ███ ███████ ██████
Summarize Argument: Phenomenon-Hypothesis
Some scientists hypothesize that the Amazon River previously flowed into the Pacific Ocean, not the Atlantic Ocean. Why? Because some freshwater fish in the Amazon are descended from saltwater fish that lived in the Pacific but not the Atlantic, and the Andes Mountains currently separate the Amazon from the Pacific.
Notable Assumptions
The scientists assume some Amazon fish are descended from Pacific Ocean fish because the Amazon used to flow into the Pacific, and not for some other reason. This means assuming ancestors of the freshwater fish entered the Amazon when it flowed into the Pacific. It also means assuming the Andes Mountains prevent any fish from traveling from the Pacific Ocean to the Amazon River on its current route.
12.
Which one of the following, ██ █████ ████████ ██████████ ████████ ██ ███████ ██ ███ ██████████ ████ ███ ██████ █████ ████ ██████ ████ ███ ███████ ██████
a
In many cases, ███████ ██ ██████████ ███ █████████ ████ ████ █████ ███████ ███████████████ ██ ███ ██ ████ █████ █ ██████ █████████
Since the Amazon fish are known to have descended from the Pacific-dwelling species, this is irrelevant. It doesn’t provide any new information suggesting the Amazon was once connected to the Pacific.
b
Most of the ██████████ ███████ ██ ███ ███████████ █████████ ████ ████ ███ ███████ █████ ████ ██ █ ██████ █████ ██ ███ █████████ ██ ███ █████ ██████████
This confirms that the Pacific-dwelling species lived before the Andes separated the Amazon from the Pacific. It rules out the possibility that the Pacific-dwelling fish emerged when the Amazon was already inaccessible from the Pacific because of the Andes Mountains.
Plausibility
Plausibility
Presenting evidence that corroborates (in Strengthen) or conflicts (in Weaken) with the author's hypothesized explanation or the predictions that follow from that explanation.
c
Many species of ████ ████ ███████ ███ ████████ █████ ███ ███████ ██ ████ ███████ ████ ███ █████ ██ ███████ ███ ███████ ██████
If anything, this weakens the argument. It suggests an alternative hypothesis: that an intermediate descendant of the Pacific-dwelling species lived in the Atlantic, and served as an ancestor for the current freshwater species in the Amazon.
d
The Andes Mountains ██████ ████ ███ ████████████ ██ ███ ████████████ ████████ ██ ███ █████ ████████ ██████████
This is irrelevant. It’s important only that the Andes cut off the Pacific from the Amazon—their precise geographical span doesn’t affect the argument.
e
There are very ███ ████ ███████ ████ ███ █████ ██ ██ ████ ██ ███████ ██ ████ █████ ███ ████ ██████
If anything, this weakens the argument. It suggests it’s unlikely that saltwater fish from the Pacific entered the Amazon directly and descended into the present-day freshwater species.