Conclusion: people who test drive Zenith car buy one coz of its impressive quality.

(A) Test-drives of Zenith cars are, according to Zenith sales personnel, generally more extensive than a drive around the block and encounter varied driving conditions. [this does not weaken the conclusion. Eliminated!]

(B) Usually dealers have enough Zenith models in stock that prospective purchasers are able to test-drive the exact model that they are considering for purchase. [how does dealers stocking cars weaken the conclusion? infact it serves to strengthen it in one way coz the dealers see to that the customer gets what he wants and hence they buy the car. Eliminated]

(C) Those who take test-drives in cars are, in overwhelming proportions, people who have already decided to buy the model driven unless some fault should become evident. [Bingo! here it states that those who want to buy the car generally test drive it, so it does not mean that whoever test drives buys the car. Weakens the conclusion.My answer]

(D) Almost 90 percent of the people who purchase a car do not do so on the day they take a first test-drive but do so after another test-drive. [whether after the first or second, in the end they do buy the car after test driving rite? Eliminated!]

(E) In some Zenith cars, a minor part has broken within the first year, and Zenith dealers have issued notices to owners that the dealers will replace the part with a redesigned one at no cost to owners.[totally unrelated answer choice. Eliminated!]

Hope this helps!

P1. 80% of the ones who test drive buy the car.
P2. if you drive a car around the block, 80% chance that you end up buying it.

CCLN Be warned you should not test drive Zenith Unless you are prepared to buy one.

And there is more to it in the question that the stimulus is interpreted as implying that the quality of the car is unusually impressive. From this we could establish a Cause and Effect relationship.

impressive quality --> 80% of the test drivers buy the car.

Now to weaken this stimulus we look at a different reason for the purchase rate.
C does this exactly by citing a different reason for this rate, that is most of the people who test drive are the ones who have decided to buy a new car. This is a different reason for the purchase rate and hence the answer.

IMO C

bcoz C directly attacks the advertisements claim that the car is so impressive during the test drive that you will end up buying it.
its not because of the car being impressive during the test drive but because of many of them having already made up a decision about it.

This one is a typical causal reasoning type(u can refer PowerScore CR for more reference on Causal reasoning & COnditional Reasoning). I suggest you always dentify a pattern & recognise that pattern as soon you skim thru the stimulus.

Pattern stated here in advt :

Here the advt says A causes B, where B is the result or effect. How can we weaken the advt claims??
@ phenoix says "alternate cause" C leads to B rather than A causing B.

But I prefer to say, this one is a cause & effect reversal rather than introduction of new alternate cause. Advt says, since

quality of the car is impressive, the sales are high. C introduces a reversal effect that "since the customers have already made a "mindset" to but Only the Zenith car,the sale of car shoots up.

If u keenly go through the bolded statements "So be warned: you should not test-drive a Zenith unless you are prepared to buy one..... Now that C reverses the case...Mind is already prepared so only the customer is doing a test drive and not the other way around. This one way of weakening the Advt claims.

But it really doesn't matter "how u r weakening the claim." but ..at the end ARE WE WEAKENING THE CLAIM" matters.

daretodream wrote:Advertisement: Over 80 percent of the people who test-drive a Zenith car end up buying one. So be warned: you should not test-drive a Zenith unless you are prepared to buy one, because if you so much as drive a Zenith around the block, there is a better than 80 percent chance you will choose to buy it.
If the advertisement is interpreted as implying that the quality of the car is unusually impressive, which one of the following, if true, most clearly casts doubt on that implication?

(A) Test-drives of Zenith cars are, according to Zenith sales personnel, generally more extensive than a drive around the block and encounter varied driving conditions.
(B) Usually dealers have enough Zenith models in stock that prospective purchasers are able to test-drive the exact model that they are considering for purchase.
(C) Those who take test-drives in cars are, in overwhelming proportions, people who have already decided to buy the model driven unless some fault should become evident.
(D) Almost 90 percent of the people who purchase a car do not do so on the day they take a first test-drive but do so after another test-drive.
(E) In some Zenith cars, a minor part has broken within the first year, and Zenith dealers have issued notices to owners that the dealers will replace the part with a redesigned one at no cost to owners.

'A' does not attack the advertisement directly.

'B' strengthens the conclusion

'C' directly attacks the advertisements claim that the car is so impressive during the test drive that you will end up buying it. It is not because of the car being impressive during the test drive but because of many of them having already decided to buy it.

'D' also does not attack the advertisement directly but says that the decision is made after another test drive and not the first one.

'E' is irrelevant to the scope of the argument.