Well, I did a little research as I always do before answering and I found that this is from CR 1000 - so not an official source and so it is difficult to verify the actual official answer.
I have generally found that people claim the answer is "E" although on the Manhattan forum the moderator agreed with "C" as the answer - although I did not see "C" mentioned anywhere else.
Even more interestingly this was posted on an LSAT forum as proof of the fact that GMAT questions are not well-written. Of course this is not an official GMAT question but I do not think it is that bad, except that neither C nor E is the correct answer.
Let's look at this one. If you read Stuart's explanation (from when this was posted a few years ago) you will that Stuart treated this as cause and effect. He said that the effect - the increase in fees and slower service - is supposed to be caused by - mismanagement. Then any answer choice that offers an alternate cause would weaken this conclusion because it would show that mismanagement is not the cause. He then explains while all except for D offer alternate causes.
Let's go through the choices and show why he was right - D is the correct answer!
To weaken the question it is necessary to drive a wedge between the evidence and the conclusion. So we are looking for reasons why the price increase and slow delivery do not neccessarily indicate mismanagement.
A) If the volume of mail is up dramatically this could account for slower delivery. This weakens the mismanagement as the cause- eliminate.
B) Fuel costs lead to increased prices. Weakens eliminate.
C) This is interesting. At least one source had this as the correct answer. However, this cannot be correct because it does weaken. It shows that there must be a different cause for the increased prices other than mismanagement. This is because the postal service charges less!! Some have said that this choice it irrelevant. But it is actually quite good evidence. Look at the wording "private delivery services usually charge more than does the postal service for comparable delivery charges." This means that on average the private services charge more. This indicates that whatever reason there is for price increases is a general one (inflation, fuel costs, insurance costs, etc.) Whatever is driving up the cost of delivery applies to all in the field and to others more than to the postal service. Could the postal service still be mismanaged and the other companies are simply worse? Yes. But remember to weaken is not to completely disprove. So this weakens the mismanagement as the cause since the postal service is cheaper than the others.
By the way "GMATMadeEasy" this is not opposite of the stimulus. The stimulus talks about prices increasing at the postal service but it makes no comparison. It is actually D that tries to negate the premises as you will see!
D) This answer does not attack the link between the evidence and the conclusion (as it should) but rather it attacks the evidence itself - never a successful strategy. We are told in the premises that there has been "an actual decrease in the speed and reliability of service." Choice D then tells us that one type of mail is actually delivered more quickly. This does not matter. If I were to tell you that, on average, the children are getting taller and so we need bigger desks you cannot negate my argument by pointing out the three shortest kids in the room! By pointing out the average I have taken the shorter children into account. The premise has also taken into account the one type of mail that is quicker - but yet (and remember that premises are always true) there has been an overall decrease in speed. So D does nothing with the argument. It attempts to weaken a premise - not allowed on the GMAT - and so it does not weaken the argument. This is the answer.
E) This is the one that has been mentioned as the answer however this cannot be. Remember that in critical reasoning words have meaning. Look at choice E very carefully "The average level of consumer prices overall has increased more than 300 percent over the last forty years." Can you spot the word that makes this choice NOT the right answer? The word is more. This would not be a great answer even without this word - as Stuart explained this answer indicates that there has been substantial inflation, which can be a cause of the increased prices. However the word "more" really insures that Choice E does weaken. This is saying that 300% is the absolute minimum of the inflation. Could it have been 500%? 1000%? Yes it could have. But it could not be less than 300% so that shows substantial inflation and that is an alternate cause for the price increase.
Many people choose E focused on the difference between 300% (minimum inflation) and 1000% (increase in postal rates). However, we do not know that inflation captures all the true costs of delivering the post so we cannot say that for sure there is a 700% gap. Add to this the word "more" than 300%. E could do more to weaken, but it does show that there are reasons for the price increase - other than mismanagement.
The fact that D attacks a premise - a fact - while C and E each weaken the link between the premise and conclusion makes D the correct answer, this is the one that DOES NOT weaken.
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