The popular belief that a snake's age can be determined by counting th

Question

The popular belief that a snake's age can be determined by counting the number of layers of scales on its body is generally true. However, to help regulate its internal temperature, the black mamba snake often sheds its outermost layers of scales when the temperature exceeds approximately 120 degrees Fahrenheit, leaving the snake with fewer layers of scales than it would otherwise have. Thus, if a black mamba snake has frequently been exposed to temperatures exceeding 120 degrees Fahrenheit, counting the number of layers of scales on its body will most likely result in an inaccurate calculation of the snake's age.
 
Which of the following is an assumption on which the argument is based?

(A) A black mamba snake must be exposed to temperatures above 120 degrees Fahrenheit for several hours before shedding its outermost layers of scales.

(B) A black mamba snake will not consistently regenerate the layers of scales it sheds after exposure to high temperatures.

(C) All black mamba snakes that have frequently been exposed to temperatures exceeding 120 degrees Fahrenheit have shed at least one layer of scales.

(D) Exposure to temperatures exceeding approximately 120 degrees Fahrenheit is the only environmental factor that triggers black mamba snake to shed its outermost layers of scales.

(E) The black mamba snake grows no more than two new layers of scales on its body each year.

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GMATNinja · Accepted Answer

On strengthen, weaken, or assumption questions, I always like to start with a nice, clear restatement of the conclusion:

"...if a black mamba snake has frequently been exposed to temperatures exceeding 120 degrees Fahrenheit, counting the number of layers of scales on its body will most likely result in an inaccurate calculation of the snake's age."

Great. And what's the reasoning the author used to reach this conclusion? We know the following:

Generally, a snake's age can be determined by counting the number of layers of scales on its body.  
 The black mamba snake often sheds its outermost layers of scales when the temperature exceeds approximately 120 degrees Fahrenheit, leaving the snake with fewer layers of scales than it would otherwise have.

So apparently, the black mamba grows layers of scales at a constant rate as it ages, but will lose some of those layers if it's exposed to high temperatures. And we're looking for something that will reinforce the conclusion that "counting the number of layers of scales on its body will most likely result in an inaccurate calculation of the snake's age."

The author's argument is based on the premise that "to help regulate its internal temperature, the black mamba snake often sheds its outermost layers of scales when the temperature exceeds approximately 120 degrees Fahrenheit." At best, (A) is irrelevant, because it wouldn't do anything to impact whether we can accurately determine the snake's age.

But you could also argue that statement (A) would actually weaken the author's argument. For example, consider the black mamba snake that has been exposed to temperatures exceeding 120 degrees Fahrenheit four times in its life. The author would expect that this snake lost its outermost layer of scales each of those four times. But what if the snake was only exposed to those high temperatures for an hour each time? If statement (A) were true, that snake would not have shed its outermost layers of scales, and counting the number of layers of scales would likely result in an  accurate  calculation of the snake's age.

Either way, choice (A) can be eliminated.

If a black mamba consistently regenerates the layers of scales that it sheds after exposure to high temperatures, then exposure to high temperatures would have no effect on the number of layers of scales that the snake has. For example, a five-year-old black mamba might normally have 10 layers of scales. If that black mamba lost four layers of scales because of exposure to high temperatures, the author would expect that snake to only have six layers of scales. But if the snake regenerated each layer of scales that it lost, then the snake would indeed have 10 layers and counting the number of layers of scales would likely result in an  accurate  calculation of the snake's age.

Thus, the author's argument relies on the fact that the black mamba will NOT consistently regenerate the scales it sheds after exposure to high temperatures. Choice (B) is therefore a necessary assumption, and it looks pretty good.

The author does not claim that exposure to high temperatures causes ALL black mamba snakes to shed at least one layer of scales. Rather, the author states that "the black mamba snake  often  sheds its outermost layers of scales when the temperature exceeds approximately 120 degrees Fahrenheit" (often, not always). Thus, the author's argument does not rely on the assumption stated in choice (C).

Because "the black mamba snake often sheds its outermost layers of scales when the temperature exceeds approximately 120 degrees Fahrenheit", the author concludes that "if a black mamba snake has frequently been exposed to temperatures exceeding 120 degrees Fahrenheit, counting the number of layers of scales on its body will most likely result in an inaccurate calculation of the snake's age." Even if other environmental factors also trigger black mamba snakes to shed their outermost layers of scales, the author's argument, which is only concerned with exposure to temperatures exceeding 120 degrees Fahrenheit, would not be impacted.

However, consider the following alternative conclusion: "if a black mamba snake has NOT frequently been exposed to temperatures exceeding 120 degrees Fahrenheit, then counting the number of layers of scales on its body will most likely result in an ACCURATE calculation of the snake's age." If the author had reached that conclusion instead, then the author's argument would have required making the assumption stated in choice (D). However, as is, the author's argument does not rely on the assumption stated in choice (D).

It doesn't matter how many layers of scales a black mamba grows each year. As long as the number of layers grown per year is known, one could theoretically calculate the snake's age by counting the number of layers. Choice (E) is not a required assumption.

Choice (B) is the best answer.

rajatgmat19 · Answer

I Chose Option D on the basis of cause and effect relation. ie.It's only to exposure to 120 degree heat that causes Black Mamba to shed it's layer's. 
120 degrees Fahrenheit is the only environmental factor that triggers black mamba snake to shed its outermost layers of scales.

If we negate doesn't the conclusion fall apart. Doesn't this option needs to be true for the conclusion to follow, Please explain?

GMATNinja · Answer

If we negate (D), then there might be OTHER environmental factors that can also trigger a black mamba snake to shed its outermost layers of scales. In that case, those other factors might also lead to an inaccurate calculation of the snake's age.

So what? Exposure to temperatures over 120 degrees will still cause the snake to shed a layer of scales, even if other factors can have the same effect. As long as high temperatures can cause the snake to shed a layer of scales, the author's logic and conclusion are still sound.

However, if the snake REGENERATES layers lost after exposure to high temperatures, then the argument falls apart.

shahMeet · Answer

B is the best choice. However, I was somewhat confused by the wording. The argument does not tell us what constitutes a high temperature, just that after 120F a particular snake performs some action. So in the answer choice how can we assume that high temperature means > 120F? Are we allowed to make such assumptions?

GMATNinja · Answer

We are told that "the black mamba snake often sheds its outermost layers of scales when the temperature exceeds approximately 120 degrees Fahrenheit."

Even though (B) itself does not tell us what constitutes "high temperatures", we know from the passage that the shedding of scales due to heat exposure happens when the temperature exceeds approximately 120 degrees Fahrenheit. In other words, (B) is simply referring to the heat-related phenomenon already described in the passage. You do not have to make any further assumptions here.

I hope that helps!

aragonn · Answer

HI  GMATNinja  Sir,

I got a bit confused with D and its negation, As you mentioned that cause of negation we can achieve what we want. which is that age will be inaccurate. Even this way I see both choices are parallel in logic. (B and D). Although i chose B, and eliminated D as out of scope, but this way it seems to be a contender.

GMATNinja · Answer

The key to choice (D) is the word "only"... If (D) is not assumed, then it is  possible  that other environmental factors cause the snake to shed.

Recall that the author's conclusion is: "if a black mamba snake has frequently been exposed to temperatures exceeding 120 degrees Fahrenheit, counting the number of layers of scales on its body will most likely result in an inaccurate calculation of the snake's age."

What if, for example, the snake also sheds when humidity exceeds 90% for 10 days straight? That would NOT impact the author's argument, which is only concerned with the effect of frequent exposure to high  temperatures .

So even if other environmental factors (i.e. humidity) cause shedding, the argument that frequent exposure to high temperatures causes an inaccurate calculation is unaffected. The argument can hold REGARDLESS of whether (D) is true, so it must be eliminated.

(B), on the other hand, is a REQUIRED assumption. If (B) is not assumed, then the snake COULD consistently regenerate the layers of scales it sheds after exposure to high temperatures. That means that frequent exposure to high temperatures would NOT impact the age calculation. The argument does NOT hold unless (B) is assumed, so it is our best answer.

I hope that helps!

Dinesh654 · Answer

Hello experts,

I eliminated B for 2 reasons,

“A black mamba”- hence I didn’t read rest of the choice carefully. 
What does this mean?
Since we have to be very precise in CR. 
Just because a black mamba regenerates its layers of scales, doesn’t mean most do.
Hence it does support - black mamba often sheds not always.

Or its talking about all? In general?

Second- what if high temperature is 200+F and the argument can still be valid for 120 to 200 F. and falls apart after 200F.

GMATNinja · Answer

For an explanation of why (B) is correct, check out our  earlier analysis  of this question, and let us know whether that clears it up!

But just in case it helps, here are a few thoughts on your specific questions.

First, when (B) says "A black mamba snake," it is making a statement about black mamba snakes in general, not one particular black mamba snake. As a similar example, consider the phrase "A new car will cost less to maintain than a used car." Although it doesn't explicitly say "all used cars," the sentence is making a statement about used cars in general.

Regarding your second point: it's true that "high temperatures" does not specify a certain temperature range. But even if "high temperatures" meant temperatures exceeding 200 Fahrenheit, the assumption in (B) would still be required by the argument.

Keep in mind that the argument concludes that the age predicted by the number of scale layers will likely be inaccurate if a black mamba snake is "exposed to temperatures exceeding 120 degrees Fahrenheit." In other words, the argument says that as long as the temperature is above 120 Fahrenheit, the scale layers method will not be accurate. But if the scale layers method  is  accurate at certain temperatures above 120 Fahrenheit (temperatures exceeding 200 Fahrenheit, for instance), the conclusion falls apart.

I hope that helps!

VerbalBot · Answer

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The popular belief that a snake's age can be determined by counting th

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