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CAT Critical Reasoning Practice question with Solution 12

The energy an animal must expend to move uphill is proportional to its body weight, whereas the animal’s energy output available to perform this task is proportional to its surface area. This is the reason that small animals, like squirrel, can run up a tree trunk almost as fast as they can move on level ground, whereas large animals tend to slow down when they are moving uphill.

Which one of the following is an assumption on which the explanation above depends?

[A]. The amount of energy needed to move uphill is no greater for large animals that it is for small animals.
[B]. Small animals can move more rapidly than large animals can.
[C]. The ratio of surface area to body weight is smaller in large animals than it is in small animals.
[D]. There is little variation in the ratio of energy output to body weight among animals.
[E]. The amount of energy needed to run at a given speed is proportional to the surface area of the running animal.
Answer: C

We’re looking for an assumption in an explanation, so your first task is to locate the explanation and the observation it is supposed to explain. The Keyword phrase “This is the reason that . . .” gives the structure away: The first sentence is meant to explain the observation discussed in the second sentence. Squirrels can run up steep inclines very quickly whereas large animals slow down while moving uphill. What is it about squirrels that gives them the edge? The explanation claims that the energy required to run uphill is proportional to body weight, but the energy available to run uphill is proportional to surface area. So the animals that have an easier time running uphill have relatively more surface area as compared to their body weight. How does this apply to the observation about squirrels? We know that squirrels weigh less than larger animals, and so they don’t need as much energy to run up hills, but we can also infer that squirrels have less surface area than larger animals, and so they have less energy available to do the job. So while squirrels have an advantage in one aspect (body weight), they have a disadvantage in another (surface area). So what else has to be true about squirrels? As (C) puts it, squirrels must have a high surface area to weight ratio, which means that they have more surface area relative to their body weight. (C) fills in the gap by explaining why the squirrels’ disadvantage in surface area is more than compensated by their advantage in body weight. Squirrels may have a little less energy to do the job, but this is more than made up by the fact that larger animals are much, much heavier.

(A) Au-contraire: In the event that large animals weigh more than small animals (which is most likely the case), the first line of the stimulus supports the opposite of (A).

(B) The issue here is moving uphill. The explanation therefore need not rely on any comparison of the general speed of small and large animals. Moreover, (B) ignores the surface area issue and thus fails to tie together the theory and the phenomenon it’s meant to explain.

(D) and (E) both focus on ratios that are never mentioned in the stimulus (energy output is proportional to surface area; energy needed is proportional to weight), so there’s no way that either of these can be the assumption on which the explanation depends. Moreover, (E) has the same problem as (A)— “run” is simply too general because the stimulus focuses on moving uphill.

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