Critical Reasoning

Human-made networks of canals and reservoirs served as irrigation systems for inhabitants of the Zana Valley as early as the fourth millennium B.C.E. These irrigation systems were constructed because large-scale agriculture would have been impossible for these inhabitants without them. The existence of similar networks in the Indus Valley in Pakistan and North India as early as 3000 B.C.E. provides strong evidence for the hypothesis that large-scale agriculture was practiced by inhabitants of the Indus Valley as early as 3000 B.C.E.

The argument outlined above relies on which of the following assumptions?


(A) If inhabitants of the Indus Valley practiced large-scale agriculture in 3000 B.C.E, some evidence of that agricultural activity could still be found today.

(B) Societies that construct irrigation systems can practice large-scale agriculture more successfully than societies that do not construct irrigation systems.

(C) All societies that practice large-scale agriculture construct irrigation networks that include canals and reservoirs.

(D) Inhabitants of the Indus Valley in 3000 B.C.E could not have practiced large-scale agriculture had they not constructed networks of canals and reservoirs that served as irrigation systems.

(E) The inhabitants of the Indus Valley would not have constructed networks of canals and reservoirs if these networks were of no use to the inhabitants of the Indus Valley.

How to use the negate the assumption technique here to find the answer? or is it feasible?

Mike,

It is a real gr8 explanation.

But as per the link given by you below for one practice question similar to this problem. I have doubt on "E" answer. E - tells about the "supreme court justices and not the policy" Can you throw some light on this.
But i would also select as by the Process of Elimination.

It is typical for courts to apply statutory or common law-as opposed to public policy-to provide a basis for their decisions. However, in a review of recent cases, it was found that the United States Supreme Court used public policy arguments to support its decisions more often than federal appellate courts used in their decisions. This indicates that public policy arguments are more effective than statutory or common law-based arguments.

Which of the following assumptions, if true, is necessary to the conclusion stated above: that public policy arguments are more effective than statutory or common law-based arguments?
a. Supreme Court justices use public policy arguments to support the majority of their decisions.
b. In the 1970s with Roe v. Wade, a case that has stood for 30 years, the Supreme Court justices demonstrated their use of public policy arguments to support their decision.
c. Appellate courts can make decisions by using policy arguments as easily as by using common law and statutory arguments.
d. Statutory and common law arguments are an inappropriate basis for deciding important legal issues.
e. Supreme Court justices are more effective at rendering decisions on important cases than their federal appellate court counterparts.

Mike@Magoosh wrote:Dear karthikpandian19,

I know some teachers and some schools are fans of the "negate the assumption" strategy. Yes, that can be helpful in some cases. I tend to think of the assumption more in terms of a "bridge" or a "link" between the evidence and the conclusion. I discuss my strategy a bit in this blogpost:
https://magoosh.com/gmat/2012/arguments- ... -the-gmat/

This argument:
KNOWN: Human-made networks of canals and reservoirs served as irrigation systems for inhabitants of the Zana Valley as early as the fourth millennium B.C.E.
KNOWN: These irrigation systems were constructed because large-scale agriculture would have been impossible for these inhabitants without them.
OBSERVATION: The existence of similar networks in the Indus Valley in Pakistan and North India as early as 3000 B.C.E.
CONCLUSION: [this system of networks] provides strong evidence for the hypothesis that large-scale agriculture was practiced by inhabitants of the Indus Valley as early as 3000 B.C.E.

First of all, those two "KNOWN" statements --- we have no idea how the author absolutely knows those are true. If this were an academic paper, we might attack those. On the GMAT CR, though, we have to simply accept all the evidence as given without questioning it. We have to accept the two KNOWN statements and the OBSERVATION as simply factual. What links these with the conclusion?

Well, the observation tells us, as a fact, that the Indus Valley folks had a network of canals. I'll shorting this to "having canal network" --- that's the observation.

The conclusion is: the Indus Valley folks practiced large scale agriculture. I'll shorten this to "big farms."

The assumption links those two, so it's something like
"having canal networks" leads to "big farms"
or, in slightly more elaborated form ...
Any civilization that has an extensive system of canals must have them for the purpose of supporting large-scale agriculture.
or in if-then form
If a civilization constructs an extensive system of canals, then it must use them to support large-scale agriculture.

That's how I build my tentative assumption. Now, I am ready to move through the answers:

(A) If inhabitants of the Indus Valley practiced large-scale agriculture in 3000 B.C.E, some evidence of that agricultural activity could still be found today.
This one starts with the conclusion, "big farm", making that the premise. That's backwards from what we need.

(B) Societies that construct irrigation systems can practice large-scale agriculture more successfully than societies that do not construct irrigation systems.
We don't care about societies that do not construct irrigation systems -- not part of the argument, not part of the assumption for which we are looking, just irrelevant.

(C) All societies that practice large-scale agriculture construct irrigation networks that include canals and reservoirs.
In if-then form, this says:
If a society practices practice large-scale agriculture, then it constructs irrigation networks with canals.
This is the converse of what we want --- if what we want is "If A, then B", then the statement "If B, then A" does not help us.

(D) Inhabitants of the Indus Valley in 3000 B.C.E could not have practiced large-scale agriculture had they not constructed networks of canals and reservoirs that served as irrigation systems.
This is also the converse --- If the Indus Valley folks practiced large-scale agriculture, then they had to build the canals. --- Again, it's (If B, then A), but we are looking for (If A, then B).

(E) The inhabitants of the Indus Valley would not have constructed networks of canals and reservoirs if these networks were of no use to the inhabitants of the Indus Valley.
This one is promising. Our tentative form of the assumption was ---
If a civilization constructs an extensive system of canals, then it must use them to support large-scale agriculture.
This is a little more broad, essentially saying
If a civilization constructs an extensive system of canals, then it must use them to do something!

This is slightly broader, but encapsulates the logic of the assumption that we constructed. Unlike (C) & (D), the logical relationships of this answer choice follow the order of the assumption we constructed above. Therefore, this is the best answer.

Does this analysis make sense? Here's another CR "assumption" question, for practice:
https://gmat.magoosh.com/questions/1319
When you submit your answer, the following page will have the complete video explanation.

Please let me know if you have any further questions.

Mike

OA is E

and OE is as follows:

The argument states that irrigation systems comprised of networks of canals and reservoirs were constructed in the Zana Valley in the fourth millennium B.C.E because large-scale agriculture would have been impossible in the Zana Valley at that time without them. Then, based on the fact that similar networks (of canals and reservoirs) existed in the Indus Valley as early as 3000 B.C.E, the argument concludes that inhabitants of the Indus Valley must also have practiced large-scale agriculture as early as 3000 B.C.E. Therefore, the argument assumes that the canals and reservoirs that existed in the Indus Valley were, like those in the Zana Valley, intended for large-scale agriculture and not some other purpose.

To determine which assumption is necessary to the argument, we should negate the assumption provided in each answer choice. If negating the statement invalidates the argument's conclusion, that statement must be assumed in order for the conclusion to hold.

Choice E is correct. If the inhabitants of the Indus Valley did construct the networks for no functional reason, then we can no longer conclude that these networks are evidence of the practice of large-scale agriculture in the early Indus Valley. The assumption in this choice, if negated, undermines the link between the discovery of the networks of canals and reservoirs and the practice of large-scale agriculture.

Choice A states that modern evidence must exist if, in fact, this civilization had practiced large scale agriculture 5,000 years ago. Even if the practice of large-scale agriculture by early Indus Valley inhabitants did not leave searchable traces, however, we could still reasonably conclude that the canals and reservoirs in the Indus Valley indicate the early practice of large-scale agriculture in that area.

Choice B introduces an irrelevant comparison. The argument assumes, based on the fact that irrigation systems were necessary for large-scale agriculture in the Zana Valley, that irrigation systems are a reliable indication of the practice of large-scale agriculture in another location (the Indus Valley). Even if at least some societies without networks of canals and reservoirs practice large-scale agriculture more successfully than societies with them, it could still be true that the discovery of irrigation networks in the Indus Valley in particular indicates the past use of large-scale agriculture in that area.

Choice C is extreme. The argument does not assume that large-scale agriculture only exists in the presence of irrigation networks; it simply makes an assumption about one particular place (the Indus Valley) in which irrigation networks perhaps indicate the existence of large-scale agriculture. Furthermore, the argument does not assume that all irrigation networks that would facilitate large-scale agriculture must specifically include canals and reservoirs.

Choice D is very tempting, but ultimately incorrect. Even if the inhabitants of the Indus Valley in 3000 B.C.E could have practiced large-scale agriculture without constructing irrigation networks of canals and reservoirs, it does not mean that they did not practice large-scale agriculture by these means. Even if this choice is not assumed, it would still be possible for the discovery of canals and reservoirs to point to the practice of large-scale agriculture in the early Indus Valley.

Choice E is correct

karthikpandian19 wrote:Human-made networks of canals and reservoirs served as irrigation systems for inhabitants of the Zana Valley as early as the fourth millennium B.C.E. These irrigation systems were constructed because large-scale agriculture would have been impossible for these inhabitants without them. The existence of similar networks in the Indus Valley in Pakistan and North India as early as 3000 B.C.E. provides strong evidence for the hypothesis that large-scale agriculture was practiced by inhabitants of the Indus Valley as early as 3000 B.C.E.

The argument outlined above relies on which of the following assumptions?


(A) If inhabitants of the Indus Valley practiced large-scale agriculture in 3000 B.C.E, some evidence of that agricultural activity could still be found today.

(B) Societies that construct irrigation systems can practice large-scale agriculture more successfully than societies that do not construct irrigation systems.

(C) All societies that practice large-scale agriculture construct irrigation networks that include canals and reservoirs.

(D) Inhabitants of the Indus Valley in 3000 B.C.E could not have practiced large-scale agriculture had they not constructed networks of canals and reservoirs that served as irrigation systems.

(E) The inhabitants of the Indus Valley would not have constructed networks of canals and reservoirs if these networks were of no use to the inhabitants of the Indus Valley.

How to use the negate the assumption technique here to find the answer? or is it feasible?

Thank you Mike

Hi Mike,
For the original question, how would we go about negating choice (D)?



Thanks!

It makes sense.

Thanks!

Elaborate explanation for negation:
The argument states that irrigation systems comprised of networks of canals and reservoirs were constructed in the Zana Valley in the fourth millennium B.C.E because large-scale agriculture would have been impossible in the Zana Valley at that time without them. Then, based on the fact that similar networks (of canals and reservoirs) existed in the Indus Valley as early as 3000 B.C.E, the argument concludes that inhabitants of the Indus Valley must also have practiced large-scale agriculture as early as 3000 B.C.E. Therefore, the argument assumes that the canals and reservoirs that existed in the Indus Valley were, like those in the Zana Valley, intended for large-scale agriculture and not some other purpose.

To determine which assumption is necessary to the argument, we should negate the assumption provided in each answer choice. If negating the statement invalidates the argument's conclusion, that statement must be assumed in order for the conclusion to hold.

Choice E is correct. If the inhabitants of the Indus Valley did construct the networks for no functional reason, then we can no longer conclude that these networks are evidence of the practice of large-scale agriculture in the early Indus Valley. The assumption in this choice, if negated, undermines the link between the discovery of the networks of canals and reservoirs and the practice of large-scale agriculture.

Choice A states that modern evidence must exist if, in fact, this civilization had practiced large scale agriculture 5,000 years ago. Even if the practice of large-scale agriculture by early Indus Valley inhabitants did not leave searchable traces, however, we could still reasonably conclude that the canals and reservoirs in the Indus Valley indicate the early practice of large-scale agriculture in that area.

Choice B introduces an irrelevant comparison. The argument assumes, based on the fact that irrigation systems were necessary for large-scale agriculture in the Zana Valley, that irrigation systems are a reliable indication of the practice of large-scale agriculture in another location (the Indus Valley). Even if at least some societies without networks of canals and reservoirs practice large-scale agriculture more successfully than societies with them, it could still be true that the discovery of irrigation networks in the Indus Valley in particular indicates the past use of large-scale agriculture in that area.

Choice C is extreme. The argument does not assume that large-scale agriculture only exists in the presence of irrigation networks; it simply makes an assumption about one particular place (the Indus Valley) in which irrigation networks perhaps indicate the existence of large-scale agriculture. Furthermore, the argument does not assume that all irrigation networks that would facilitate large-scale agriculture must specifically include canals and reservoirs.

Choice D is very tempting, but ultimately incorrect. Even if the inhabitants of the Indus Valley in 3000 B.C.E could have practiced large-scale agriculture without constructing irrigation networks of canals and reservoirs, it does not mean that they did not practice large-scale agriculture by these means. Even if this choice is not assumed, it would still be possible for the discovery of canals and reservoirs to point to the practice of large-scale agriculture in the early Indus Valley.

Choice E is correct.

ice_rush wrote:Hi Mike,
For the original question, how would we go about negating choice (D)?



Thanks!

Mike@Magoosh wrote:Dear karthikpandian19,

(D) Inhabitants of the Indus Valley in 3000 B.C.E could not have practiced large-scale agriculture had they not constructed networks of canals and reservoirs that served as irrigation systems.
This is also the converse --- If the Indus Valley folks practiced large-scale agriculture, then they had to build the canals. --- Again, it's (If B, then A), but we are looking for (If A, then B).

This is the essence of the trap! Missed noticing it. Thanks for the wonderfully effective explanation Mike!

PS: I'm a big fan of your OG SC videos. They are short and effective! I think you MUST upload videos for the supplementary SC questions as well.