A scientist made three observations: (1) in the world's temperate zones, food is more plentiful in the ocean than it ...

Ravi March 26, 2019

@Steph,

Happy to help. This question has quite a bit going on in it!

The scientist's hypothesis in the stimulus is the argument's main
conclusion: food availability is a determining factor in the migration
of migratory fish. The scientist arrived at his conclusion by
analyzing migratory temperate water fish. He found that they spawn in
fresh water, and make their homes in the ocean, where they have access
to more food. We know from the stimulus that in tropical waters, the
opposite phenomenon takes place. In comparing the migratory fish of
tropical waters to temperate waters, we know that using the
scientist's 2nd criterion, they have opposite tendencies for the 2nd
criterion, and the scientist's 3rd criterion applies to all types of
migratory fish. What we don't know, however, deals with the first
criterion. While we know that that oceans in temperate areas have more
food than fresh water, what about in the tropics? In the tropics, is
there more food in the ocean, or is their more food in fresh water?

The question is asking us to choose an answer choice that highlights a
question that would be most helpful to help us evaluate the argument.
On these types of questions, it's best to think of what gaps exist
between the premises and the conclusion of the argument. In order for
the question in the answer choices to be correct, answering it one way
should make the argument stronger, and answering it the other way
should make it weaker. The correct answer choice must contain a
question that does both of these things depending on what the answer
to the question is. If the answer to the question only strengthens or
weakens the argument when it's answered one way—but doesn't do
anything when it's answered the other way—then we know that we have a
poor choice. As noted above, the scientist's 1st criterion is the
question we don't have an answer to, and this is probably what we're
looking for to be phrased a question in the answer choices.

(A) says, "whether in the world's temperate zones, the temperatures of
bodies of fresh water tend to be lower than those of the regions of
the oceans into which they flow"

The problem with (A) is that the argument is all about how available
food is to these fish. We don't care about the temperatures of the
ocean waters and their relationship to the temperature of the waters
to where they flow.

(B) says, "whether the types of foods that migratory fish eat while
they inhabit the ocean are similar to those that they eat while they
inhabit bodies of fresh water"

We aren't concerned with the type of food, as the scientist's
hypothesis is all about the availability of food. Type of food isn't
mentioned anywhere in the stimulus.

(C) says, "whether any species of fish with populations in temperate
zones also have populations that live in the tropics"

This is a tricky answer choice. On one hand, if the answer to this
question is "yes," then it adds confusion by adding uncertainty to
whether or not food is a key factor in migration. If the answer to the
question is "no," then the answer doesn't do anything for us because
we have no new information that helps us conclude anything new.

(D) says, "whether there are more species of migratory fish in the
tropics than there are in temperate zones"

We aren't concerned with the number of species. Even if there were
more species in the tropics, we don't know that means in terms of
whether or not they make a decision on their migration based on how
available food is to them.

(E) says, "whether in the tropics food is less plentiful in the ocean
than in fresh water"

(E) is right on the money. If the answer to (E) is "yes," then we know
that the migratory patterns of tropical fish match the patterns of
temperate fish (the fish spawn where there's not much food and make
their homes where there's lots of food). However, if the answer is
"no," then the scientist's hypothesis is weakened. This is our correct
answer choice.

Does this make sense? Let us know if you have any more questions!