#9 - Rodents are small, gnawing mammals characterized by the

[Dave Killoran]

Complete Question Explanation

Must, #%. The correct answer choice is (B).

This problem requires that students read very closely, as the author shifts terms in the second sentence.

The stimulus is a Fact Set which begins by noting the nature of rodents. That is then followed by a statement that in North America most species are not rodent species but that most individual mammals are rodents. At first, many readers stop because this sentence appears to contain a contradiction. How can most mammals not be rodents but then most mammals are rodents? The key is to realize the first reference is to species and the second to individual animal numbers. So, while the majority of species might not be rodents, it still can be that the majority of individual mammals are rodents. But the only way for that to occur is for the numbers of rodents to be relatively high, which is stated in the correct answer choice, (B).


Answer choice (A): This answer is incorrect due to the inclusion of "species." If "Species of" were removed, this answer would be correct.

Answer choice (B): This is the correct answer choice. As discussed above, this follows from the last sentence.

Answer choice (C): We do not have any information about the mammal species with incisor teeth in the rest of the world, and thus we cannot make a comparison, and this answer is incorrect.

Answer choice (D): This answer has some similarities to (A). While this answer is possible, we cannot be certain is undeniably true. There might be a single non-rodent species with many members, but then collectively the rodent species still outnumber that species (and all other non-rodent mammals). But that does not have to be the case either, making this answer incorrect.

Answer choice (E): This answer has some similarities to (C). We do not know anything about the numbers of species of non-rodent mammals in North America except that they outnumber the rodent species. Thus we cannot draw a universal conclusion about whether most of them that exist in the world can be found in North America.

[juandresmc]

Hey PS!

Could you please explain what is the difference between answer choice (B) and (D)? During my practice test I ended up choosing (D) but didn’t know why was a “better” answer.

Thanks in advance!

Regards,

Andrés

[Luke Haqq]

Hi Andrés!

Happy to address why answer choice (B) is a better one than (D). First, it's important to identify this as a must be true question, and additional one that involves numbers/percentages/proportions.

Second, since the stimulus is notably short and doesn't seem to contain any complex topics or new terms, pay extra close attention to the language being used. As the explanation in the previous post notes, the second sentence both refers to "mammals" and to "mammal species." Regarding mammals, the stimulus explains that most of them in North America are rodents. Regarding mammal species, however, it is clear that most mammal species in North America are not rodent species.

How can most of the mammals in North America be rodents, while at the same time most mammal species in North America are not rodent species? One could imagine that the continent is home to a very large number of rodents--say this number is into the billions--yet only from a few different rodent species. By contrast, the number of other non-rodent species like cats, dogs, sheep, pigs, etc. could be in the thousands. In such a hypothetical, one would have a case in which most of the mammals are rodents (since there are assumed to be billions of them) while at the same time most species are not rodent species (thousands of non-rodent species, compared to a few rodent species).

Answer choice (D) states, "Of the mammal species in North America, the one with the most individual members is a species of rodent." The words I have italicized makes this answer problematic. We can't be sure of this based on the stimulus alone. The claims that the stimulus makes are about rodents generally (which, the stimulus indicates, encompasses multiple species of rodents). We know from the stimulus that "most of the individual mammals in North America are rodents" generally, but we are not told more about the prevalence of particular species of rodents. For the sake of illustration, suppose that the billions of rodents fall into a million different rodent species; though this is extreme, it at least exemplifies how one could have a large number of rodents overall in North America, while at the same time not having any individual species of rodent being the one mammal with the most species. If there were millions of different rodent species as in this example, then it is possible that another, non-rodent species could outnumber any single rodent species even if not outnumbering rodents as a whole. Since this answer choice does not have to be true, it cannot be correct on a must be true question.

In contrast, answer choice (B) correctly uses the term "species" in line with the information provided. It is a statement about "rodent species" as a whole, which is the correct reference point: "In North America, rodent species tend to have more individual members than other species of mammals have." We know this to be true based on the stimulus because it states that "most of the individual mammals in North America are rodents."

[JocelynL]

Hello,

When I came across this stimulus I thought it was formal logic and diagrammed it as:

North America mammal species Not rodents

Individual mammals in North America rodents small gnawing mammals with chisel like incisor teeth.

In reading a/c B I thought it was trying to combine the two statements above and didn't think it was correct. Can someone help me realize when formal logic is present and when its not?

Is diagramming in this one really not helpful to get to the correct a/c and instead we need to pay attention to the wording in the stimulus and pick up on the difference between the "species" of mammals vs "individual" mammals within the species group?

[JocelynL]

I forgot to add, I chose a/c D because I thought it was a restatement of the second diagram

[Robert Carroll]

Jocelyn,

Your diagramming is perfect, and this stimulus did indeed have formal logic present. However, as you recognized, it's not particularly helpful for this one. And that's ok! It's good to recognize formal logic whenever you see it, and if you are familiar enough with it to create a diagram quickly and efficiently, it won't consume too much time even when it's not necessary for understanding the stimulus.

Answer choice (B) is not a flawed combination of the two statements, but one that must be true. If rodents are a minority of species, but a majority of individual animals, then we have the following numbers:

North American mammal species that are rodents = relatively small = A

North American mammal species that are not rodents = relatively large = B

Number of individual North American mammals that are rodents = some other relatively large number = C

Number of individual North American mammals that are not rodents = some other relatively small number = D

"Relatively small" and "relatively large" are specific to the pairs they are in. So B is bigger than A. C is bigger than D. But no other numerical comparisons are possible or intended.

Answer choice (B) is saying that the number of individual rodents per species is bigger than the number of individual non-rodent mammals per species. This is reducible to the following:

(C divided by A) is bigger than (D divided by B).

And this has to be true. C is bigger than D, and B is bigger than A. So (C divided by A) is bigger than (D divided by B) because it has a bigger numerator and smaller denominator.

Answer choice (D) is not a restatement of the stimulus. Look at its use of the word most: "the one with the most individual members." By illustration, consider the following two statements:

"I had most of the cookies you made last night."

That requires me to have more than 50% of the cookies.

"Of all the people in the restaurant, I was the one who ate the most biscuits."

That does not require me to eat more than 50% of the biscuits. It requires me to eat more biscuits than anyone else. But I could eat 5 biscuits, my friend 4, and my girlfriend 3. I ate 5 of 12 biscuits, so fewer than half, but I still ate "the most biscuits."

Answer choice (D) is using "the most" in that way. The stimulus is using "most" in the way you diagrammed, which is my first example's (the cookie example's) way.

Robert Carroll

[JocelynL]

Thanks so much. Makes perfect sense. I also realized the first sentence is completely irrelevant.

[Tajadas]

I was stuck between B and D, but eliminated D because there is no evidence to support that the one species with the most individual members is a rodent species.

Looking at B now though-- B doesn't have to be true, does it? What evidence is there to support it? Maybe there is one rodent species with a huge number of individual members, but there are more rodent species (e.g. 2 species) with lower numbers of individuals than other species of mammals have. I think this example would still be valid according to the text and would disprove B, because the rodent species don't "tend" to have more individual members than other mammals, it's just one species that does.

With this in mind, why is B more easy to support than D, given that neither must be true? Is it just because you need to make a bigger assumption for D to be true than for B to be true?

[Adam Tyson]

With this in mind, why is B more easy to support than D, given that neither must be true? Is it just because you need to make a bigger assumption for D to be true than for B to be true?

That's exactly the issue! Keep in mind that this question stem is the softer variation of Must Be True, a Most Strongly Supported question. That means the answer doesn't absolutely have to be true, but is a reasonable inference with at least some support from the stimulus.

For answer D I would say there is no support for it. The math tells us about averages across all species, but nothing about any one species. It is entirely possible that the one species with the most individuals is a non-rodent, maybe some kind of cat for example.

But the math requires that the average number of rodents per species must be higher than the average number of non-rodents per species - there's no getting around that. Fewer species but more individual members must equal a higher average number per species. And while that average could be artificially inflated by one or two super-populous rodent species, that requires working pretty hard to disprove the answer, when instead we should just be asking ourselves "is there at least some support for this answer?"

Our reaction to answer B should be "seems right," but our reaction to D should be "well I don't know about that, now, do I?" That's enough to make B the better choice. To put a twist on the way you said it, any problem with B requires a bigger assumption, or more work, than the problem with D. D needs more help, so it's worse.