Urban air contains more sulfur dioxide than does rural air, and plants

Question

Urban air contains more sulfur dioxide than does rural air, and plants in cities typically grow more slowly than do plants in rural areas. In an experiment to see how much of the difference in growth is due to sulfur dioxide, classes in an urban and a rural school grew plants in greenhouse at their schools and filtered the greenhouse air to eliminate sulfur dioxide. Plants in the urban greenhouse grew more slowly than those in the rural greenhouse.

Which of the following, if true, would it be most important to take into account in evaluating the result?

(A) The urban school was located in a part of the city in which levels of sulfur dioxide in the air were usually far lower than is typical for urban areas.

(B) At both schools, the plants in the greenhouses grew much more quickly than did plants planted outdoors in plots near the greenhouse.

(C) The urban class conducting the experiment was larger than the rural class conducting the experiment.

(D) Heavy vehicular traffic such as is found in cities constantly deposits grime on greenhouse windows, reducing the amount of light that reaches the plants inside.

(E) Because of the higher levels of sulfur dioxide in the air at the urban school, the air filters for the urban school’s greenhouse were changed more frequently than were those at the rural school.

KarishmaB · Accepted Answer

Responding to a pm: LEt's understand the argument first: There are two facts given: 1. Urban air has more sulphur dioxide. 2. Urban plants grow slower. We want to find the effect of high sulphur dioxide in the air on the slow growth. Say, urban plants grow at the rate of 2 cm per month. Rural grow at 8 cm per month. If we remove sulphur dioxide from air, we might see that urban plants grow at 6 cm per month while rural grow at 10 cm per month. This will show us the effect of higher sulphur dioxide. But these numbers will be dependable when everything else is kept the same. Only then we can separate out the contribution of higher levels of sulphur dioxide. If the urban greenhouse is different from the rural greenhouse, the difference could account for a part of the difference in growth rate too. Option (D) tells us that urban greenhouse gets lesser light. This means we cannot measure the contribution of higher sulphur dioxide to slower growth rate. Hence we need option (D) to evaluate the result. (A) The urban school was located in a part of the city in which levels of sulfur dioxide in the air were usually far lower than is typical for urban areas. This is irrelevant. Note that sulphur dioxide was filtered out already by both schools. So now the levels of sulphur dioxide make no difference to the result of the experiment. (B) At both schools, the plants in the greenhouses grew much more quickly than did plants planted outdoors in plots near the greenhouse. We want to compare rural greenhouse to urban greenhouse. Not within greenhouse to outside. (C) The urban class conducting the experiment was larger than the rural class conducting the experiment. Class size is irrelevant. (D) Heavy vehicular traffic such as is found in cities constantly deposits grime on greenhouse windows, reducing the amount of light that reaches the plants inside. Correct. (E) Because of the higher levels of sulfur dioxide in the air at the urban school, the air filters for the urban school’s greenhouse were changed more frequently than were those at the rural school. It doesn’t matter how frequently the filters were changed. The end result was that all sulphur dioxide was filtered out. Answer (D) Discussion on Useful to Evaluate Questions: https://youtu.be/1JtHjH1lWZc

gixxer1000 · Answer

I would say D.

The experiment is set up to compare urban and rual areas. Greenhouses are set up in both rual and ubran areas to remove the sulfur dioxide to see what portion of the differnce in growth is actually due to presence of sulfur dioxide. For this experiment to work all other element should be the same.

The greenhouses in the urban areas have grime on the windows reducing the amount of sunlight the plants get. This is the most important thing to know because it tells us that the experiment is not equal and we cannot attribute the results just to the removal of the sulfur dioxide.

Then urban greenhouse plants may grow slower than the rual greenhouse plants becaue they naturally grow slower or they may grow slower because they are getting less sunlight. Either way the addition of D tells us the results of the experiment are not valid.

AlbertNTN · Answer

OA is D, thks for the explanation

I chose B because i think that proves sulfur dioxide affects growing speed of the plants (remove the air makes it grow faster, then in rural when it is less, plants grow faster also). Still don't know how to eliminate this... or might be i infer too much ...

b. At both schools, the plants in the greenhouses grew much more quickly than did plants planted outdoors in plots near the greenhouse

gixxer1000 · Answer

Answer B is only looking at the results.

"Which of the following, if true, would it be most important to take into account in evaluating the result?"

If you have an experiment and I ask you what should be the most important thing you should consider to evaluate the results you shouldn't say the results themselves. You should be focused on the criteria of the experiment to make sure it's fair.

yavasani · Answer

UAir (SO2) > RAir 
Cities (plant growth) < Rural 
Exp: U & R school grew plants in green houses and filtered SO2.
Results : Urban Plant growth < Rural plant growth.

The filtering process has any issues ? Or any other outside factors taking effect ?
Are we comparing apples to apples (same plants ? , same climatic conditions etc) ?

crackHSW · Answer

IMO it should be E and not D , if excluding the SO2 from the experiment , Urban plants still growing slowly than rural ones then it is sure that SO2 is not playing a role here and there are some other factors , however the fact is that the guy cnducting the experiment already knows this(he has observed this) and he/she wants to be absolutely sure that there are no other factors contributing to this so he would evaluate things which strenghten his belief that So2 has been eliminated properly.

animanga008 · Answer

I was split between B and D on this one.

B is suggesting that something in addition to SO2 was affecting plant growth in urban areas but nonetheless SO2 is indeed a factor. I think this ties together the effect the question is trying to determine.

D is directly saying that because urban greenhouses received less light, they grew less which implies that the plants in urban areas would have grew like the ones in rural areas had it not been for grime.

I feel as if B is implying other variables while D is directly stating it...perhaps thats why D is a better choice.

KarishmaB · Answer

I am not sure how you figured that B is suggesting "that something in addition to SO2 was affecting plant growth in urban areas ".
Actually B is suggesting that sulphur dioxide does slow down plant growth substantially. When plants were grown in a greenhouse where the air was filtered to remove sulphur dioxide, their growth rate increased in both urban and rural areas. 
In fact, in (B), they have mentioned "than did plants planted outdoors in plots near the greenhouse" specifically to tell us that the conditions were very similar other than the fact that greenhouse air had no sulphur dioxide. 
This is the result of the experiment. It isn't something necessary to evaluate the result.

imhimanshu · Answer

Hello Experts,
I am having a hard time understanding this argument.

The argument says that they want to check the effect of SO2 on the growth of plants grown respectively in urban and in rural areas. 
Now, the students are doing an experiment in green houses and they have filtered the S02. Now, isn't it the case that the plants should grow at equal rates because the only thing that hampering their growth was SO2. However, they have found that the difference in growth is still there. How come this be possible.

In nutshell, isn't the experiment itself is erroneous. that is, if you want to see the impact of sulphur dioxide on the growth rates of plants,and you are removing the SO2 from the atmosphere, then how can you judge the impact.

Please help.

Thanks
imhimanshu

mikemcgarry · Answer

Dear Himanshu, I'm happy to help. :-)

I don't know how much you have studied experimental design or have been involved in the design of scientific experiments. Demonstrating correlation is a very very easy thing to do --- one experiment, or one observational study, is enough. Demonstrating causality, by contrast, is a monstrously difficult task --- sometimes it takes years, or even decades. For example, in establishing the link between cigarette smoking and cancer, the correlation was well measured from the late 1950's onward, but conclusive evidence of causality was available only in the 1990's, only after molecular biology had made substantial advances. Suffice it to say: one never can conclude causality from only one experiment. In studying the effect of SO2 on plants, a causality question, there are several questions to answer, and this experiment is a very well designed experiment to test one set of those questions. True, it's not the only experiment we could conduct, and if we really were interested in rigorously demonstrating causality, we would have to do a wide variety of experiments, some of which involved absolutely bombarding plants with SO2. This experiment, eliminating SO2 from the controlled environment, is one valid experiment, just not the only possible one. If SO2 and only SO2 were the difference, then we would expect the plants with no SO2 to grow at exactly the same rates in both places. NEVER be so literalist as to think that an experiment testing for one factor won't unexpectedly be influenced by another factor . That happens all the time in experimental design --- you try to control for everything, and sometimes that works, but sometimes a factor you never even considered turns out to make the crucial difference. Some great discoveries in the history of science have happened precisely in this way. In this experiment, we eliminate the SO2, and the plant growth is still different, which lets us know --- in addition to the SO2 levels, there's also something else that affects the growth of these plants. Then, the important question becomes --- what is that "something else", and (D) provides a possible answer to that question. Does all this make sense? Mike :-)

imhimanshu · Answer

Thanks Mike for the detailed explanation.
I would have answered this question had the question been Explain the paradox; The paradox being that despite controlled atmosphere, students found the difference in growth rates. And the answer Choice D would be apparent.
But, for Evaluate the argument, I usually apply "Variance Analysis" i.e testing the extremes and then analyzing the behavior of the argument.

Applying variance analysis:

Lets say that Yes. heavy vehicular traffic deposited grime on greenhouses windows, and reduced the light. -> It could be a possible factor in the difference of the growth rates of the plants.

However, if you say that there is no such grime deposited on greenhouse gases, and possibly the amount of light is same in greenhouses, then it is not describing the behavior of the growth rates. 
Here, I am getting confused, if it is not the cause of difference in growth rates, then what could possibly be the factor behind the growth rates.

Another doubt is that I have seen evaluate questions comes in two varieties -
First is in which the answer choices starts with "Whether, Would , Were" etc that converts it into a form of question. In such questions, I can easily apply variance analysis.

However, second categories have plain answer choices such as the answer choices in this question.. then is it correct to apply Variance analysis here, or what should be the appropriate strategy.

Please comment.

Thanks

mikemcgarry · Answer

Dear Himanshu, I'm happy to respond. :-)

To be perfectly honest, forget "variance analysis". Forget any pre-fab, one-size-fits-all rule. The GMAT excels at creating diverse questions ---- each new questions depends on a logical twist unique to that particular situation and unlike anything in any other question. if you try to apply fixed rules to the GMAT CR, it will fool you time and time again. You must forget all general methods, and dive into a critical analysis of what is unique and particular about the individual situation at hand. In Evaluate the Argument questions, a correct answer need only have impact on the argument if the question is answered one way. Suppose there's a GMAT CR argument, and it's a "evaluate the argument" prompt. Suppose the OA is: " Whether P causes Q ?" Now, suppose that, if P does cause Q, that would significantly change the argument, but if P does not cause Q, that would have zero effect on the argument. That's perfectly fine. That's the norm for Evaluate the Argument CR questions. The OA may pose a question that has crucial implications for the prompt argument either way, but that's not necessary ---- as long as one way of answering that question has crucial implications for the argument, then it doesn't matter whether the other answer would not affect the argument at all. Does all this make sense? Mike :-)

oishik · Answer

If you assume that heavy vehicle emit sulfur dioxide and cause grime, only then D will be correct :D
idk whether you can assume upto that

mikemcgarry · Answer

Dear oishik, I'm happy to respond. :-)

Think about the structure of the question. The prompt question says: Which of the following, if true , would it be most important to take into account in evaluating the result? In other words, the test is telling us to assume that each one of the answer choices is true. For each answer choice, our job is to assume that the information in the answer is 100%, and then see if this new information sheds light on the argument. When you are considering answer choice (D), not only are you allowed to assume that it is true, but you are supposed to assume that! The question explicitly tells us that it is your job to assume that (D) is completely true, for however long you are considering that particular choice as a possible answer. You don't even get a choice about this: you have to assume that (D) is true to understand this answer choice. Does this distinction make sense? Mike :-)

akhileshdas · Answer

I came up with the assumption : Plant growth rate is not affected by Sulphur Dioxide at all and is affected by some other reason. Choice D gave me that reason and hence I selected it.  egmat  can you comment if my assumption is correct? I assume is it since I was able to select the right choice and none of the other choices are close to the assumption I came up with.

mikemcgarry · Answer

Dear akhileshdas , I'm happy to respond. :-)

My friend, I would say that you reasoning is excellent. Don't doubt yourself! That was an intelligent and perfectly valid approach to this question. Mike :-)

Abhishek009 · Answer

Same with me , :oops:

I started doubting other factors affecting Plant growth ( One of the factors in my list was soil texture ), however I was content with option (D) putting the blame on the Heavy vehicular traffic. :-D

OreoShake · Answer

This question uses the word 'evaluate' in the question stem, but is really a resolve the paradox question. tag should be changed, how do we do that?

sayantanc2k · Answer

The tag is alright. Whether option D was taken into account while arriving at the conclusion is the point. It's not that the experiment poses a paradox and then option D explains the paradox. If option D was taken into account then the result of the experiment is correct, if option D was NOT taken into account, then the result may be faulty. Hence the question is an "evaluate argument" type question.