Many gardeners believe that the variety of clematis vine that is most

The conclusion of this passage is that the gardener's belief--that jackmanii is the most popular variety of clematis vine among gardeners in North America (NA)--is apparently correct. The word "apparently" is very important here. The conclusion would be substantially different if we were to replace "apparently" with "definitely".

More on that in a moment... but first, why does the author conclude that the belief is apparently correct?

• The largest clematis nursery in NA sells one million clematis plants per year.
• Of the one million clematis plants sold per year by that nursery, ten percent are jackmanii.

A substantial chunk of the clematis plants sold at the largest nursery are jackmanii. The author reasons that jackmanii's popularity at the largest nursery is a good indicator of its popularity among NA gardeners.

But what about the other 90%? What if another variety of clematis accounts for MORE than 10% of the one million? In that case, jackmanii would not be the most popular variety at the nursery. However, if no other variety accounts for 10% (or more) of the one million, then jackmanii would be in fact be the most popular variety at the nursery.

Would that prove that jackmanii is the most popular variety in NA? Not necessarily. But if jackmanii is #1 at the largest nursery, that's certainly EVIDENCE that jackmanii is #1 in NA. Again, we are trying to conclude that the belief is APPARENTLY correct, not that the believe is DEFINITELY correct. As long as we have evidence that jackmanii is the most popular variety among NA gardeners, we're in good shape.

With that in mind, let's look at the answer choices:


If the nursery sells 10 varieties or fewer, then there would HAVE to be another variety that accounts for more than 10% of the one million plants sold at the nursery. If that were true, then jackmanii could not be the most popular variety at the nursery. Thus, we would not have evidence that jackmanii is #1 in NA. Without choice (A), the argument falls apart.

Now, does choice (A) PROVE that jackmanii is #1 at the nursery? Not at all... even with 20 varieties, there could be another type that accounts for more than 10% of the million. That, of course, would ruin the argument.

Even if choice (A) is true, the argument may or may not be valid, but that's okay. In other words, choice (A) doesn't PROVE that the author's reasoning is sound. But without choice (A), the author's argument could not be valid. This is a required assumption, so keep (A).


The nursery could sell many other types of plants. As long as jackmanii is the most popularity variety of clematis plant sold at the nursery, then the author's reasoning holds up. This is not a required assumption, so eliminate (B).


If a substantial portion of jackmanii sales were to gardeners outside of NA, then we might have a problem. That might suggest that jackmanii's popularity at the nursery is NOT a good indicator of jackmanii's popularity among NA gardeners.

Choice (C) only says that SOME of the jackmanii are sold to gardeners outside of NA. In that case, the jackmanii sales at the nursery are probably a good indicator of its popularity among NA gardeners.

Does that make (C) a REQUIRED assumption? If (C) were not true and the nursery ONLY sold jackmanii to gardeners within NA, then that would probably further strengthen the argument. That would give us even more reason to believe that the sales at the nursery are a good indicator of NA popularity. Choice (C) is not a required assumption, so eliminate (C).


It doesn't matter whether clematis is a popular plant in general. The belief is that jackmanii is the most popular variety OF clematis. Even if only a tiny fraction of NA gardeners grow clematis, jackmanii could still be the most popular variety among those gardeners who DO grow clematis. Eliminate (D).


This is admittedly a little bit tempting: sure, it would strengthen the argument. But this isn't a strengthen question: we need to know whether this is NECESSARY to draw the conclusion.

And it isn't necessary: even if (E) is NOT true -- for example, if jackmanii accounts for less than 10% of clematis plants at a few nurseries -- it's still possible that jackmanii is the most popular. And since (E) isn't necessary, it's not the correct answer.

(A) is the best answer.
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Hi,

there has been a discussion on the validity of A as the answer.

PARA says that of all the plants sold, 10% are jackmanii and Jackmanii is the most popular among gardeners..

A tells us--
a.The nursery sells more than 10 different varieties of clematis ..
yes you can say that it does not guarantee that jackmanii is the most popular, It is possible that with even 12 varieties, there is some variety that may be more than 10%..
But 'Assumption' is something without which the argument CANNOT stand..
this is exactly the same happening here..
Is it possible that varieties are less than 10 and still jackmanii with 10% is the most popular-- NO
Because there will be atleast one variety more than 10% in that case and Jackmanii will not be the most popular..

So this is a VALID assumption..
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The best way to answer this question is via negation technique.

A states - "The nursery sells more than 10 different varieties of clematis"
Negate A: The nursery sells less than 10 different varieties of clematis. --> Out of 1 million if less than 10 varieties are sold, then there is atleast one variety ( >10% popularity) which is more popular than Jackmanii (10% popularity) --> Conclusion breaks down.

A is the correct answer.

Many of the explanations here are completely correct, so I'm not sure that I have anything astoundingly new to add to the discussion. But we've had a few requests for expert help, so I'll give this a whirl.

For Arpit, among others: I think that part of the confusion is that we're looking for an assumption that's necessary. That doesn't always mean that it's going to be 100% airtight, or that you'll be able to reach the conclusion with 100% certainty based on that assumption. But the correct answer will be something that's needed to draw the conclusion, even if it's not enough by itself.

In other words: without that assumption, the conclusion couldn't hold.

I see lingering doubts in the thread about why (E) is wrong and why (A) is right.

Let's start with (E):


This is admittedly a little bit tempting: sure, it would strengthen the argument. But this isn't a strengthen question: we need to know whether this is NECESSARY to draw the conclusion that "the variety of clematis vine that is most popular among gardeners in North America."

And it isn't necessary: even if (E) is NOT true -- for example, if jackmanii accounts for less than 10% of clematis plants at a few nurseries -- it's still possible that jackmanii is the most popular. And since (E) isn't necessary, it's not the correct answer.

So what about (A)?



This NEEDS to be true. If the nursery sells fewer than 10 different varieties, then there's no way that jackmanii -- with a market share of 10% at the biggest nursery -- could possibly be #1. This is absolutely necessary to draw the conclusion -- and therefore our correct answer.

But again, (A) isn't necessarily sufficient by itself, either: you could have 20 varieties, and jackmanii could, in theory, be beaten out by one or more of them. But you still need to assume that there are more than 10 in order for jackmanii to be #1. The key takeaway: an assumption can be 100% necessary (and therefore the correct answer), while still leaving possibilities for the conclusion to be false.
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All credits to CR Bible: This is an excerpt of my personal review session. Hope it helps:

First of all – before starting keep in mind that there are two types of assumptions: i) supporter assumptions and ii) defender assumptions. Let’ s provide an example of both:

i) Support assumption are assumptions that link together new elements in the stimulus or fill logical gaps in the argument. An example of this would be:

All male citizens of the Netherlands have the right to smoke weed. Therefore, Riccardo has the right to smoke weed in the Netherlands. The supporter assumption here is that Riccardo is a male citizen.

The fact that we assume Riccardo is a male citizen contains a piece of information not previously mentioned in the stimulus. In this case, Riccardo is a new element in the conclusion. This creates a gap in the conclusion. Therefore – the supporter assumption plays a paramount role in bringing these two terms together. Another example of this supporter assumption is:

All ducks in Martinique that have a really big beak are treated as holy and can therefore not be eaten by the local residents of Martinique. Therefore, Duckie, my favorite duck that I visit every day can’t be eaten by the local residents of Martinique.

The new element in the argument here is Duckie! The supporter assumption links this new piece of information together with the previous element. By assuming that Duckie has a really big beak.

ii) Defender assumptions are assumptions that contain statements that eliminate ideas or assertions that would undermine the conclusion. In some way, they defend the argument by showing that a possible source of attack is eliminated. Let’s take an example of this:

People who exercise a lot more perform better academically. Thus, exercising must cause a person to improve his/her grades. The author assumes here that the following assumptions aren’t true:

i) Sleeping more than eight hours a day doesn’t improve one’s grades.
ii) A high protein diet doesn’t enable someone to study more effectively and therefore improve their grades.

These hidden assumptions protect the argument against statements that would undermine the conclusion. Simply put, the author assumes that every alternate cause doesn’t exist!

Now let’s pivot back to the exercise at hand:

Many gardeners believe that the variety of clematis vine that is most popular among gardeners in North America is jackmanii. This belief is apparently correct since, of the one million clematis plants sold per year by the largest clematis nursery in North America, ten percent are jackmanii.
Which of the following is an assumption on which the argument depends?

(A) The nursery sells more than 10 different varieties of clematis
(B) The largest clematis nursery in North America sells nothing but clematis plants
(C) Some of the jackmanii sold by the nursery are sold to gardeners outside North America
(D) Most North American gardeners grow clematis in their gardens
(E) For all nurseries in North America that specialize in clematis, at least 10% of the clematis plants they sell are jackmanii.


Reviewing the stimulus we see:

Conclusion: The most popular variety clematis vine amongst gardeners in North America is jackmanii.

Premise: Of the one million clematis plants sold per year, by the largest clematis nursey in North America, ten percent are jackmanii.

Prephrase:
The new element in this stimulus here is the largest clematis nursery in North America. Remember that a supporter assumption brings together two pieces of new information. This would imply that the pieces sold at the largest clematis nursey are representative of the rest of the country. So we need a supporter assumption!

Let’s review the answer choices with this prephrase in mind:

a) The nursery sells more than 10 different varities of clematis. This would mean that the nursery indeed, is representative of the entire clematis vine population. If they would just sell two species, this would imply that that the jackmani is the least popular. This answer choise directly links the premise with the conclusion of the argument. CONTENDER!

b) Ok – if it only sells clematis plants that’s great for them. But does it really tell us anything about the variety of clematis vine? This answer choice is tricky, as it requires us to specifically understand what is being said. The word variety is key.

c) Some… This would merely imply that it is popular outside North America – but the conclusion focusses on inside North America. Out of scope.

d) Great for them – what is most? This would strengthen it in some way. But is it the underlying assumption that holds the premise and conclusion together? No. Because if most NA gardeners grow it in this garden, this doesn’t tie together the element regarding the largest clematis nursery and the conclusion. Remember that this is an assumption question and not a strengthener.

e) This was my first answer choice – as it slightly matched my prephrase, but it’s wrong because: we still don’t know the amount of varieties each nursery holds. And therefore, doesn’t logically link the premise with the conclusion. Because it could still mean that the other nurseries sell only two/three varieties. Let’s label it as a contender for now.

Deciding between the two contenders A and E

Let’s use the assumption negation technique to decide between the two contenders. The assumption negation technique simply determines whether the negated answer choices weaken the argument, and therefore help identify the lynchpin of the argument - the assumption that holds the premise and conclusion together: the central nervous system, heart or brain whatever metaphor we want to use.

The assumption negation technique consists of two steps:

i) Logically negate the answer choices under consideration.
ii) The negated answer choice that attacks the argument will be the correct answer.
Understand that the consequence of negating an assumption is that the validity of the conclusion is called into question. So when we take away (negate it) the assumption – the building block of the argument – the entire reasoning structure is called into question.

Let’s negate:
a) The nursery doesn’t sell more than 10 different varieties of clematis.

b) For all nurseries in North America that specialize in clematis, at least 10% of the clematis plants they don’t sell are jackmanii. So 90% sold aren’t jackmani.

Which one of these weakens the argument?

If the nursery doesn’t sell more than 10 different varieties of clematies, this would mean that the 10% sold is actually less than the majority. If it sells 10 varieties for example, it is a mere 10% of the other varities, which would directly attack the premise and make weaken the argument the most. Therefore answer A is the right choice between A and E. For E – this would still mean that 10% sold are Jackmani and therefore doesn’t weaken the argument at all and is therefore not a supporter assumption that links the two pieces of information together.

So answer choice is A: for the simple fact that it’s a support assumption and when negated, the argument doesn’t hold, and we can therefore conclude that A is the entire lynchpin of the argument holding the premise and conclusion together.

Please provide kudos if this was helpful!

As explained in the explanation above, we do not need to PROVE that jackmanii is the most popular variety in NA or even that it is the most popular variety at the nursery. But if jackmanii is NOT the most popular variety at the nursery, then the author's argument falls apart.

If the nursery sells fewer than ten varieties, then there is no way that jackmanii is more popular than all of the others. At best, if there are exactly ten varieties, then jackmanii might be tied for #1. And if there are fewer than 10 varieties, then there MUST be other type that accounts for more than 10% of sales.

It is absolutely true that even with more than 10 varieties, jackmanii might not be #1. But in order for jackmanii to be #1, you MUST assume that there are more than 10 varieties. Choice (A) does not prove that the conclusion is true, but the conclusion CANNOT be true unless (A) is assumed. That makes (A) a required assumption.
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Variety of Clematis vine that is MOST popular is jackmanii in NA.
This is correct since in NA, 10% of clematis plants sold are jackmanii.

We are looking for an assumption. The following things are going through my mind.

Just 10% of clematis is jackmanii but it is the most popular variety. That means no other variety accounts for 15% or 20% of clematis sold. All other varieties account for less than 10% each. So A, a variety of clematis could account for 4%, B could account for 9% etc, but none above 10%.
This certainly means that there are more than 10 varieties to make up the 100%. If there were exactly 10 varieties, the minimum that each would need to be is at 10% to make up 100% but that would mean that all are equally popular. So there MUST be more than 10 varieties and each must account for LESS THAN 10% of the total.

Also, I am thinking, that I need to compare jackmanii with other clematis varieties. Anything that compares clematis with other plants is irrelevant. Anything that compares jackmanii with other plants (other than clematis) is irrelevant.

(A) The nursery sells more than 10 different varieties of clematis

This is an assumption. It NEEDS to be true for the conclusion to hold as discussed above.

(B) The largest clematis nursery in North America sells nothing but clematis plants

Not needed. Even if they sell other plants, that is irrelevant. We need to focus on how many varieties and in what number are clematis plants sold.

(C) Some of the jackmanii sold by the nursery are sold to gardeners outside North America

We don't need this to be true. We don't need that some jackmanii should be sold outside NA.
Do we need all jackmanii to be sold within NA? Not necessary but we would consider other information in that case.

(D) Most North American gardeners grow clematis in their gardens

We don't need this to be true. Among those who grow clematis, jackmanii is the most popular. It is possible that only 2% of NA gardeners grow clematis.

(E) For all nurseries in North America that specialize in clematis, at least 10% of the clematis plants they sell are jackmanii.

No. We have data about the largest nurseries in NA. Nurseries that specialise in clematis could be just 1% of all largest nurseries with very low volume. Their numbers could be insignificant.

Answer (A)
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Video solution from Quant Reasoning starts at 43:42
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Conclusion says:

Jackmanii is most popular of all variets of clemantis. to proove this argument tells us that 10% of all clemantis are jackmanii

Lets say we have 100 plants sold in month. Now if 10 are jackmanii there is possibility that there exists another variety that is sold 50% or more then our conclusion goes down. To prevent conclusion agains this criticisism we need to say that theier exists atleast 10 variety of Clementis.

Option E: this can be falsified: what if there are only two varieties of clementis. then other varitey will be 90%. moreover we are concerned with overall sales of jackmanii So their can be another case where one of stores sells none and other sells more than 15%. So average again comes down to 10%

Your reasoning is correct -- just because some variety of clematis is the most popular clematis at some particular nursery, it isn't necessarily the "most popular among gardeners in North America." In other words, there is definitely a hole in this argument.

But the question isn't asking whether there's a hole in the argument (which again -- there definitely is). The question is asking "which of the following is an assumption on which the argument depends."

So we're looking for something that is necessary for the argument to hold up. However, that doesn't mean it will prove the argument correct. In this case, actually, the right answer definitely doesn't prove that the conclusion is correct. But it is necessary for the argument to hold up.

For a more detailed analysis of this issue, plus an explanation of why (A) is correct, check out this post, and let us know whether that clears it up!


If there were exactly ten varieties sold, then the 10% sales of the jackmanii could NOT represent the largest percentage. And if the sales of jackmanii could not represent the largest percentage, then the argument falls apart.

To see why this is the case, consider the instance where exactly ten varieties of clematis are sold. Given this fact, can we imagine a scenario where 10% is the highest percentage?

To achieve this, we'd want the percentages for each of the other varieties of clematis to be as small as possible. And to do that, we'd want to divide the remaining 90% equally among the remaining 9 varieties. Doing this, we'd get that each variety accounts for 10% of the sales, so jackmanii would not be the most popular. In fact, it would be tied for first place with 9 other varieties.

As a follow up -- what if some of the remaining 9 varieties represented less than 10%? For instance, what if the first eight accounted for only 1% in total? Then, to bring the total to 100%, the ninth one would have to account for 89%, which would make it far more popular than the jackmanii.

In short, no matter what scenario you imagine, if there are exactly 10 varieties of clematis, the jackmanii can't be the most popular. So the argument falls apart, and (A) is correct.

I hope that helps!
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Many gardeners believe that the variety of clematis vine that is most popular among gardeners in North America is jackmanii. The statement talks about a belief held by many gardeners. What is the belief? Jackmanii is the most popular type of clematis vine among gardeners in North America.
This belief is apparently correct since, of the one million clematis plants sold per year by the largest clematis nursery in North America, ten percent are jackmanii. The author agrees with the belief based on some evidence.
‘apparently correct’: this phrase indicates that the author accepts the belief to be correct based on some piece of information.
What is the piece of information? That 10% of the clematis vines sold by the largest nursery (out a total of 1 million clematis vines sold) are jackmannii.

Author’s logic:
10% of the large number (1 million) of clematis vines sold by the largest nursery are jackmanii (basis). Therefore, jackmannii is the most popular vine of any gardeners in North America.


Gap(s) in logic:

- What’s the big deal with 10%? The author must believe that no other clematis vine sold more than 10%. There could be a vine that constituted more the 10% of the sales.
- Smaller nurseries might overshadow the largest nursery’s sales.
- Does overall sales indicate popularity among gardeners? Maybe architects order plants for landscaping without inputs from gardeners.

There could be additional gaps in the argument. I came up with these.



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

We’re looking for an assumption. ‘an assumption on which the argument depends’: this wording implies that we are looking for a statement without which the argument would not make any sense. Thus, one test we can apply while going through the answer choices is: does the argument still remain valid if we add the negation of the answer choice to the context. If it does, then the argument would not be dependent on the answer choice, and thus the choice would be incorrect. Also, an assumption is something that the author would have needed to make the argument. So, assumptions support the argument. If we find an answer choice that weakens the argument, it cannot be an assumption.

Prediction:
The author assumes that:

10% is the highest portion
The sales of the largest nursery are representative of overall sales
Popularity of a vine among gardeners can be determined on the basis of sales in nurseries

A. The nursery sells more than 10 different varieties of clematis
Correct. What is the author’s logic?
Since 10% of the vines sold by the largest clematis nursery were jackmanii the most popular such vine is jackmanii.
When would 10% be a big deal?
When no other vine sold 10% or more.
Now, if the nursery sells 10 or fewer varieties (negation), then 10% jackmanii sales would not be the single largest proportion. So, the author’s logic falls flat.

For the author to conclude that jackmanii is the most popular vine variety among gardeners on the basis the 10% figure of the largest nursery, the author must have assumed that 10% is the highest proportion of sales by any clematis vine variety.

Let’s use a bit of math to understand why this answer choice is correct.

Say, the nursery sold 9 varieties of clematis vines. Even if each variety sold equally, the proportion of sale would be 11.11% each. If any variety sold more than the others, its proportion would be even higher than 11.11%. And if there was actually a vine which was responsible for >10% of the sales, then the author would not have concluded that jackmanii is the most popular on the basis of its 10% sales figure.

So, had there been 10 or fewer clematis vine varieties sold at the nursery, at least one variety would have sold a proportion higher than 10%. That would destroy the argument’s logic.

Why does the author believe that jackmanii is the most popular clematis vine on the basis of the 10% figure? The author must assume that 10% is the highest proportion among all clematis vines. That would only happen if the total number of vines is more than 10.

Note: I find many test-takers eliminate this answer choice in their first-go, and only come back to it after eliminating all the others. To understand why this answer choice is correct requires a deeper level of quantitative reasoning than most CR questions. (I talk more about this answer choice in the additional notes at the bottom.)


B. The largest clematis nursery in North America sells nothing but clematis plants
Incorrect. Statement: The nursery does not sell any plants other than vines.

If this statement were true, then maybe the nursery is indeed a reliable source to understand which vine is the most popular vine among gardeners. i.e., the answer does support the argument mildly. However, for the argument to hold true, is it necessary for the nursery to not sell other plants? What if it did? What if the nursery did sell plants in addition to clematis vines (negation)? Even then, as long as it is the largest clematis vine nursery in North America, the argument’s logic remains intact.

It helps to be clear that the 10% sales are of ‘the one million clematis plants sold per year’ and not of the total number of plants sold per year by the nursery.



C. Some of the jackmanii sold by the nursery are sold to gardeners outside North America
Incorrect. What impact does this answer choice have on the argument? The conclusion of the argument is that that the belief that jackmanii is the most popular clematis vine among gardeners in North America is correct. If some of the jackmanii sold by the nursery are sold to gardeners outside North America, my belief in the conclusion reduces. This answer choice weakens the argument, thus cannot be an assumption.

We can also evaluate this answer choice by checking whether it is necessary for the argument. What if none of the jackmanii sold by the nursery is sold to gardeners outside North America (negation)? In that case, our belief in the argument goes up. The negation of the statement supports the argument instead of breaking it down. Thus, this answer choice is incorrect.

I find that many people reject this answer on the basis of the word ‘some’.

Let’s replace the word ‘some’ with the word ‘most’.

Most of the jackmanii sold by the nursery are sold to gardeners outside North America.

Does this change make this answer choice correct?

If most jackmanii sold by the nursery are exported outside NA, the argument gets weakened. We would then believe less in the point that jackmanii is the most popular clematis vine in NA.

If a statement weakens the argument, it can’t be an assumption. The author would not have assumed something that weakens the argument.

Thus, the reasoning that this answer choice is not an assumption because it mentions the word ‘some’ is flawed. ‘some’ or ‘most’, the answer choice weakens the argument, and thus is not an assumption on which the argument depends.


D. Most North American gardeners grow clematis in their gardens
Incorrect. The argument is based on the nursery’s sales.

Gardeners could buy from a nursery and grow clematis in their gardens, or they could plant from scratch and grow clematis in their gardens. In the former case, I do not learn anything about categories within clematis. Thus, the statement becomes irrelevant for the argument. In the latter case, the argument actually gets weakened. If most gardeners plant clematis from scratch in their gardens, the sales data of the largest clematis nursery would not really mean much with regards to which clematis vine is the most popular among gardeners.

Thus answer choice either is irrelevant or weakens the argument. Either way, it is not necessary for the argument that most gardeners grow clematis in their gardens. Even if most didn’t (negation), the argument’s logic still remains intact.



E. For all nurseries in North America that specialize in clematis, at least 10% of the clematis plants they sell are jackmanii.
Incorrect. Tricky one. This answer choice does strengthen the argument. If for all clematis-specializing-nurseries, at least 10% of the clematis plants sold are jackmanii, I certainly start to believe more that jackmanii is the most popular clematis vine variety among gardeners in North America.

Is the statement necessary though?
Is it absolutely necessary for the argument that all ‘specialist’ nurseries sell such a proportion of jackmanii?
What if even one nursery (perhaps a small one) sells a smaller proportion of jackmanii than 10% (negation)? Does the argument break down then? No, it doesn’t. The argument still makes sense even if few specialist nurseries sell less than 10% jackmanii.

Remember, we are looking for an assumption on which the argument depends.

Does the argument depend on this answer choice?
In other words, is it absolutely necessary for all such nurseries to at least 10% of their clematis sales through jackmanii?
No.




FAQ – related to answer choice A:
Even if there are more than 10 varieties of clematis vines, it isn’t necessary that 10% is the highest share. Another vine might still have sold more. Then shouldn’t option A be wrong?
There is a flaw in this reasoning. When we are looking for an assumption, we are looking for something that is necessary for the argument to make sense – i.e., without which the argument should not make any sense. However, we are not looking for something that makes that argument logically foolproof. In other words, an assumption would be necessary for the argument, but it need not be sufficient.
The above-mentioned reasoning simply states that we may not be able to reach the conclusion even if we add option A to the argument. And that’s fine. Option A is necessary – i.e., if there are 10 or fewer varieties in the nursery, then the argument’s logic gets shattered. Option is not enough (sufficient) – i.e., even if option A is true, it is possible that the conclusion may still not follow.
This distinction between necessary and sufficient is quite helpful in dealing with assumption questions. I see test-takers make two kind of mistakes in this context:

1. Eliminate the correct answer because it isn’t sufficient, even though it is necessary
2. Select an incorrect answer because it is sufficient (i.e. it confirms the conclusion), even though it is not necessary

The main point of the argument is as follows:

The belief is based on the fact that among all varieties of clematis, the jackmanii is the variety that is sold the most by the largest nursery.

Now if jackmanii constitutes 10% of the total sales by the nursery, then there must be more than ten varieties sold by the nursery - if there had been 10 or less no. of varieties sold, then there would have been at least one other variety that was sold 10% or more and thus jackmanii would not have been the one that was sold the most by the largest nursery.

The conclusion is that the belief--that jackmanii is the most popular variety of clematis vine among North American gardeners--is apparently correct. In other words, the evidence in the passage should support the belief of those gardeners. This does not mean that the evidence in the passage has to prove that the belief is accurate.

The conclusion is that the belief is apparently correct, not that the belief must be true. So, even if we could come up with scenarios that would make that belief inaccurate, the reasoning and conclusion in this passage would not be impacted.

This sets us up for choice (A), as described below: https://gmatclub.com/forum/many-gardene ... l#p1834817
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aditya201819, try reviewing these two posts:

1) https://gmatclub.com/forum/qotd-many-ga ... l#p2023640
2) https://gmatclub.com/forum/qotd-many-ga ... l#p2037613

Yes, even if there are fewer than ten varieties, jackmanii COULD still be the most popular in NA. And even if there are MORE than ten varieties, it might not be. But that's not the point. The question does NOT ask, "Which of the following must be true for jackmanii to be the most popular in NA?". The question asks, "Which of the following is an assumption on which THE AUTHOR'S LOGIC depends."

The author's conclusion is NOT: "Jackmanii is DEFINITELY the most popular in NA." The author's conclusion is that the belief is apparently correct. In other words, the author believes that the sales data from the nursery is EVIDENCE suggesting that the belief is true. Whether jackmanii is ACTUALLY the most popular is irrelevant.
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Note that an assumption is a necessary condition; it need not be a sufficient condition. You NEED more than 10 varieties but just having 11 varieties may not be ENOUGH. You need to ensure that each variety other than jackmanii is less than 10%.
The question is looking for an assumption. Option (A) is a NECESSARY condition.
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If there are only 9 varieties and jackmanii represents 10%, that means that the other 8 account for 90% (of clematis plants sold per year by the largest clematis nursery in North America).

If that's the case, at least one of those 8 would have to represent more than 10% of the plants sold. That's more than jackmanii, so the argument would fall apart.

This post discusses (A) in further detail. I hope that helps!
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Popularity of a product depends on how many of it are sold.

Say plant A and plant B.
If 100 units of plant A are sold and only 10 units of plant B are sold, which is more popular? Plant A.
Now, does it matter whether 1 person bought all 100 but 10 different people bought the 10 units of plant B? No. The 10 different people wanted only one unit of plant B and overall only 10 units of plant B were required.
Whereas, 100 units of plant A were required - whether bought by 1 large nursery owner or 50 different people.
That is true for popularity of any product - how much is it sold?
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No, jabhatta2. F is still not sufficient, because we don't know what's happening in all the other nurseries in North America. But A is even more insufficient.
Generally speaking, GMAT assumptions are almost never sufficient.

The word "depends" implies that we're looking for a necessary assumption. In theory, that necessary assumption might happen to also be sufficient, but that's extremely rare on the GMAT.

Here's an analogy:
x^2 > 25, therefore x>10
Which of the following is an assumption on which the argument depends?
(A) x is positive

A is correct, even though it's still entirely possible that the conclusion is wrong. Why, then, is A correct?
Well, if we were to discover that x is not positive, then the logic of the argument becomes nonsensical. The only reason someone concluded that x>10 is because they assumed that x lives to the right of zero. Discovering that it doesn't renders the argument's logic ridiculous.

Similarly, in the original problem, the author concludes that the variety of clematis vine that is most popular among gardeners in North America is jackmanii. Based on what? Well, based on the fact that of the one million clematis plants sold per year by the largest clematis nursery in North America, ten percent are jackmanii.
Are we supposed to be impressed by this statistic? 10% doesn't sound like a winner to me.
Indeed, if we discover that the nursery sells no more than 10 different varieties of clematis, 10% would for sure be a small fraction, relatively speaking. There would have to be another variety that outsells jackmanii. This is a quantitative reasoning inference, and it renders the argument's logic ridiculous.

In option A- The nursery sells more than 10 different varieties of clematis. Suppose, the nursey sells 12 varieties. Of the 12 varieties, suppose 10 varieties are sold at 5 % each, jackmanii at 10% and the other one at 40%. So, howcan we say that Jckamnii si the most popular considering that the sales of someone else is greater than 10%.Also, nowhere in the premise it's given that 10% is the highest sale.