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Alright, so we have a Strengthen with Necessary Premise question here. Remember that a necessary premise is said to be necessary for a conclusion if and only if the non-occurrence of the premise guarantees or brings about the non-occurrence of the conclusion. We are looking for the minimum requirement for the argument to fall apart. So when negated, the correct answer will make the stimulus fall apart. The conclusion of the stimulus is "This [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] is the reason that small animals...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." (A) is incorrect because it is irrelevant. It does not strengthen the author's argument because the argument is not just about the amount of energy needed, but rather the amount of energy needed vs. the amount of energy available. Let's look at its negation: The amount of energy needed to move uphill is greater for large animals than it is for small animals. This does not make the stimulus fall apart. If anything, it actually goes a long with what is stated in the argument. A larger animal has more body weight and therefore it would take more energy for the large animal to move uphill than for the small animal will less body weight. (B) is incorrect because it does not strengthen the argument. Who cares if small animals generally more rapidly than large animals? We are trying to explain why small animals move uphill as fast as they move on ground level, whereas large animals tend to slow down when they are moving uphill. This argument is not a comparison between the speed of small animals vs. large animals. The comparison is between small animals moving uphill vs. on ground level and large animals moving uphill vs. ground level. Let's look at its negation: Small animals cannot move more rapidly than large animals can. This does not destroy the argument because it is still possible that small animals move as rapidly uphill vs. ground level and large animals slow down while moving uphill, so the argument is still sound. (C) is CORRECT because it not only strengthens the argument but also is necessary to the argument. Let's look at its negation: The ratio of surface area to body weight is not smaller in large animals than it is in small animals. We know from the stimulus that the animal's energy output available to perform a task is proportional to its surface area and that the energy an animal must expend to move uphill is proportional to its body weight. The stimulus then concludes that this is why smaller animals can run up tree trunks almost as fast as they can move on level ground, whereas large animals tend to slow down when they are moving uphill. However, if it were true that the ratio of surface area to body weight in large animals is not smaller than that in small animals, then their tendency to move slowly when they are going uphill cannot be explained by the body weight/surface area theory. Thus, the conclusion would fall apart. This means that the argument depends on answer choice (C). (D) is incorrect because it does not strengthen the argument. Let's look at its negation: There is not little variation in the ratio of energy output to body weight among animals. The stimulus is differentiating between small and large animals. Information given to us about animals in general is not information that the stimulus depends on. (E) is incorrect because it does not strengthen the argument. Let's look at its negation: The amount of energy needed to run at a given speed is not proportional to the surface area of the running animal. Just as in (D), information given to us about animals in general is not information that the stimulus depends on.

I still do not understand why we are assuming that energy is related to speed. If C were false, it would just mean that small animals have less energy remaining when climbing uphill than big animals would. The body weight/surface area theory, as explained, just accounts for the energy of the animals, not the speed. Thus, changes in the energy of the animals does not necessarily affect speed. I'm not saying E is right; I'm saying C is not right either. Can someone explain to me what is wrong in my thinking?

Why has this not been answered this is ridiculous do your jobs please

Could you kindly explain what is the necessary assumption in this argument that the author fails to mention? In the lessons it was explained that assumptions are gaps and without them the arguments are flawed because the author tends to jump from a stated premised to the conclusion. My problem with this particular question is that this jump is not apparent to me. I'm not able to detect the missing minimum requirement and going through the answer choices one by one hasn't clarified this for me. I understand that the argument supports the conclusion that large animals tend to slow down (because of body weight and hence more energy needed) but the surface ratio part i can't fathom at all. What is it about surface ratio between large and small animals that is key here? Can you please clarify?

@nelson and @alymathieu, The stimulus tells us that 1) the energy an animal must expend to move uphill is proportional to its body weight and 2) an animal's energy output available to perform this task (move uphill) is proportional to its surface area. Given the information provided, we know that energy (capacity for doing work) is related to speed, as speed is a measurement of the animal's ability to perform the task of moving uphill over a period of time (speed is simply distance divided by time). Let's look at some examples of what the author is describing in the stimulus: 1) the energy an animal must expend to move uphill is proportional to its body weight In words, a 10 lb. squirrel expends 1/10th the energy of moving up a 100-yard hill as a 100 lb. dog would in moving up the same hill. 2) an animal's energy output available to perform this task (move uphill) is proportional to its surface area. O.K., got it. Now, let's look at the conclusion. Conclusion: This is the reason that small animals, like squirrels, 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. In order for a 10 lb. squirrel to move up the hill faster than the 100 lb. dog, what's the necessary assumption? In other words, what has to be true? Answer choice (C) is the assumption that must be true in order for this argument to hold. The ratio of surface area to body weight must be smaller in large animals than it is in small animals. Why? well, let's take a look at the negation and what the implications would be if the negation were true: Negation: The ratio of surface area to body weight is not necessarily smaller in large animals than it is in small animals. So we could have a 100 lb. dog with a LARGER surface area to body weight ratio than a 10 lb. squirrel. If the negation were true, we could theoretically have a 100 lb. dog with a surface area to body weight ratio of 20:1, and the 10 lb. squirrel could have a surface area to body weight ratio of 1:1. If this were true, then the 100 lb. dog could theoretically run up the hill faster than the squirrel because its energy output available to run up the hill would be 20 times that of the squirrel. This would totally wreck our argument, and this is why (C) is the correct answer. @Golnar-Roughani, Necessary assumptions can sometimes be difficult to spot. The truth is that in any argument, there is a multitude of necessary assumptions. For example, some other necessary assumptions that exist in this argument are that 1) animals can move and 2) gravity exists. If either of those weren't true, this argument would be wrecked. Our task is identifying which of the five answers is a necessary assumption in the argument. It's important to keep in mind that the correct answer is but one of many necessary assumptions that are required in order for the argument not to fall apart. Of the five answer choices, there is only one necessary assumption, and it's answer choice (C). The reason that (C) is a necessary assumption is because if it's false, the argument falls apart. Answers (A), (B), (D), and (E) are all assumptions that don't necessarily have to be true. If we negate each of these answers, the argument doesn't necessarily fall apart. If you guys have more questions, please let us know. We're here to help!

Thanks, Ravi. This was a great way of thoroughly explaining this.

How do you know from the passage that a larger surface area would be better for climbing uphill? I thought a smaller one would be better.