Two centuries ago, Tufe Peninsula became separated form the mainland,
isolating on the newly formed Tufe
Island a population of Turfil sunflowers. This population’s
descendants grow to be, on average, 40 centimeters
shorter than Turfil sunflowers found on the mainland. Tufe Island is
significantly drier than Tufe Peninsula was.
So the current average height of Tufe’s Turfil sunflowers is
undoubtedly at least partially attributable to changes
in Tufe’s environmental conditions. Which of the following is an
assumption on which the argument
depends?
A. There are no types of vegetation on Tufe Island that are known to
benefit from dry conditions.
B. There were about as many Turfil sunflowers on Tufe Peninsula two
centuries ago as there are on Tufe Island
today.
C. The mainland’s environment has not changed in ways that have
resulted in Turfil sunflowers on the mainland
growing to be 40 centimeters taller than they did two centuries ago.
D. The soil on Tufe Island, unlike that on the mainland, lacks
important nutrients that help Turfil sunflowers survive
and grow tall in a dry environment.
E. The 40-centimeter height difference between the Turfil sunflowers
on Tufe Island and those on the mainland is
the only difference between the two populations.

Here the answer is C.. But my doubt is if I negate the answer choice
in C."The mainland’s environment has (not) changed in ways that have
resulted in Turfil sunflowers on the mainland".

It strengthens the argument that the flower grew taller as a result of
the environment. It does not attack the argument. Could you please
throw some light on this?

Regards,
Sangeetha

Conclusion- the current average height of Tufe's Turfil sunflowers is undoubtedly at least partially attributable to changes in Tufe's environmental conditions.
This means that the changes in the newly formed island's climate caused the sunflower plants to get shorter.

We have to assume here that there were no changes on the main Island.

if you negate C, it means the climate on the mainland changed too. How will the conclusion or evidence hold true in that case? It could have become wetter which made the plants on the main island grow more and not the ones on the island grow less.
If climate at mainland changed then change in condition on Tufe island will not be contributed to small length of sunflowers. Something else will

That's right Rajan.

Just to clarify, the conclusion is not
the current average height of Tufe’s Turfil sunflowers is
undoubtedly at least partially attributable to differences between environmental condition on the mainland and on Tufe
or even
the current average height of Tufe’s Turfil sunflowers is
undoubtedly at least partially attributable to Tufe’s environmental conditions
but
the current average height of Tufe’s Turfil sunflowers is
undoubtedly at least partially attributable to changes
in Tufe’s environmental conditions

Neither of the false conclusions would be weakened if C were negated, but the real conclusion would be weakened.

Hi,

I also wanted to understand why is "D" wrong?

The difference in soil contents could also be a possible cause for this difference, right?

Thanks,
Vishal

assumptions are things that are necessary to the argument, not just things that "could".

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Ron
Option D:-"The soil on Tufe Island, unlike that on the mainland, lacks important nutrients that help Turfil sunflowers survive and grow tall in a dry environment"

Option D clearly tells that Soil on Tufe Island is not suitable to grow Tall in a Dry environment.

Be careful Vijay on what you are trying to accomplish. The question states:

Which of the following is an assumption on which the argument depends?

As Ron said above, assumptions are things that must be true. You are assuming that (D) is true, but that's not your job in this case. What if (D) wasn't true? Could you still make the argument that the height of the sunflowers has changed as a result of the environment? Sure. Maybe something else, such as water fall or animal populations caused the change.

(D) could be true.
(C) must be true, otherwise the conclusion does not stand. That's why (C) is the right answer.

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Manhattan GMAT Instructor

Hi Joe -

Is the below reasoning correct to eliminate D

Conclusion says -

So the current average height of Tufe’s Turfil sunflowers is undoubtedly at least partially attributable to changes in Tufe’s
environmental conditions.

The above bold part in the conclusion allows other causes also to account for changes in the height. Hence D cannot be assumptions.

No. Even if the bolded part of your conclusion were removed, the answer would still be (C). The conclusion is that changes in Tufe's environmental conditions caused (at least partially) the average height of Tufe's sunflowers to be different than on the mainland. The unstated assumption is that changes in the mainland's environmental conditions didn't cause those flowers height to be different.

Flower A & B used to be the same height 200 years ago

Today A is taller than B

If my conclusion is that B's conditions must have changed, my assumption is that A's conditions didn't.

_________________
Joe Lucero
Manhattan GMAT Instructor