At the end of 1997 several nations stated that their oil reserves had

Hello Guys,

I didnt get how B is the answer, can anyone explain?

The passage says that oil reserves gradually drop as old oil fields are drained and rise suddenly as new oil fields are discovered. If oil fields are being drained and new oil fields are being discovered, oil reserves would only remain the same if the amount of oil drained was EXACTLY equal to the amount of new oil discovered. The author suggests that this would be an unlikely coincidence. Thus, according to the argument, if a nation states that its oil fields have not changed over the course of year, it is likely that the nation is making an incorrect statement.

But what if over the course of a year, NO oil fields are drained and NO new oil fields are discovered? In that cause, the oil reserves WOULD remain the same. According to the author, the likely explanation for no change in oil reserves is incorrect reporting. However, another explanation is that no oil fields were drained or discovered. Thus, in order for the author's argument to hold, we must assume that the latter explanation is not likely.


Choice (B) represents this required assumption.


As for (D), as stated below, the author's argument does not RELY on this assumption. For example, some of those countries may have drained oil fields WITHOUT discovering new ones, decreasing their oil reserves. If those countries stated that their oil reserves had not changed since the end of 1996, they would still be incorrect.

In other words, those countries could still be incorrect even if choice (D) is not true.

Dear GMATNinja,

I get your explanation to some extent but not totally.
First of all the author says " the fact that the oil reserves remain changed is incorrect ".

What is the author assuming for this to be true?

Assumptions .

There was only drainage but no discoveries - thus change is inevitable ( "old oil fields were drained ")
There was only discoveries of reserves but no drainage.- thus change is inevitable (" new oil fields were discovered")
There was both drainage and discoveries - how does this signify that change was inevitable ("both")

Edit :
Just got it I think. Although there was both drainage and discoveries , the rate of both was not equal hence change is inevitable.

Can somebody explain why option B is correct?

See Gmatninja's explanation above. You probably won't find a better guy to explain something to you. I am happy to discuss this more if needed.

Yes I did go through his explanation, and I am not doubting his explanation by any means.

Still, I am having a hard time differentiating option B from D because as far as I understood, the author assumes that because new oil fields are discovered and the old oil fields are drained, there will be some change in the oil levels, therefore nations stating that their oil fields have not changed are incorrect.
But option B states that the nations drained their old oil fields, discovered the new oil fields or both, and so does option D, that nations dumped their old oil fields and discovered the new ones. So, how are these two options different?

This is where I am confused. Or I don't know, may be I am a little slow in understanding or dissecting these two options to get to the correct answer.

There is a very subtle difference but D says "If a nation incorrectly stated at the end of 1997 that its oil reserves had not changed since the end of 1996, then during 1997 that nation drained its old oil fields and discovered new ones." "And" is the keyword in D. You don't need both to happen together, and even if one of them didn't happen, the nations could still be incorrect.

Hope this helps!

Thankyou brian123. yes , this helps. Now I realize where I went wrong.

At the end of 1997 several nations stated that their oil reserves had not changed since the end of 1996. But oil reserves gradually drop as old oil fields are drained and rise suddenly as new oil fields are discovered. Therefore, oil reserves are unlikely to remain unchanged from one year to the next. So most of the nations stating that their oil reserves were unchanged are probably incorrect.

Which one of the following is an assumption the argument requires?

(A) For any nation with oil reserves, it is more likely that the nation was mistaken in its statements about changes in its oil reserves than that the nation's oil reserves remained unchanged. - WRONG. Nations mistaken is irrelevant.

(B) It is likely that in 1997, in most of the nations that stated that their oil reserves were unchanged, old oil fields were drained or new oil fields were discovered, or both.

(C) During the course of 1997, the oil reserves of at least one nation not only gradually dropped but also rose suddenly. - WRONG. It tries to base an argument only on one exceptional case.

(D) If a nation incorrectly stated at the end of 1997 that its oil reserves had not changed since the end of 1996, then during 1997 that nation drained its old oil fields and discovered new ones. - WRONG. Not necessarily true.

(E) If a nation 's oil reserves change from one year to the next, then that nation is obligated to report the change correctly. - WRONG. Irrelevant.

B is such that it would have happened normally which is what it makes it difficult. Still unbelievable that it is the answer.

Hi GMATNinja,
"or both" at the end got me. if the country both drained and found new reserves, this leave a chance that those would even out the difference. So basically, the oil reserves can be unchanged.
If there were not "or both" I would go with this answer for sure.

and also, could you please indicate how does "The author suggests that this would be an unlikely coincidence."

In regards to the "both": we're looking for an assumption that the argument requires in order for the conclusion to have a shot at holding true. That's different than an assumption that guarantees that the conclusion will definitely hold true.

So, is it possible that the info in (B) doesn't guarantee the conclusion? Sure! It could be that the draining and discovery of oil perfectly cancel one another out. That's ok, though -- we don't care whether the assumption guarantees the conclusion.

The author bases his/her reasoning on the fact that draining/discovering oil fields makes it unlikely that oil reserves remain unchanged. So, we NEED to assume that most of the countries in question are actually doing these activities. If you take out the "both," then this assumption is no longer a required assumption -- it's possible that these countries both drain and discover oil fields and their oil reserves DO change, so they're mistaken when they report no change.

(B) doesn't guarantee that the conclusion holds true, but it is required for the argument to have a chance at holding up. That's why (B) is the correct answer.

As far as the likelihood that draining and discovering oil perfectly cancel each other out: the author discusses both of these activities, and based on these factors says "oil reserves are unlikely to remain unchanged from one year to the next." So, the author implies that it isn't likely for a country to engage in these activities and end up with a perfectly balanced oil reserve.

I hope that helps!

it is one those questions where you get your answer through some good pre-thinking, works better as it is an assumption question and as you read through the question you can think of why the oil reserves remained unchanged and if the author assumed that it did not happen.
Pre thinking here- maybe reserves did rise or fall but new and old fields balanced it out and made the actual level the same.
B) directly attacks this assumption
D) is wrong because it is more detailed and the rise and fall is not assumed to have happened in that specific order.