I fell for C as well...I understand why B is correct.

B

+1 for B. Nice question. It was a close call between B and E. But the statement "You must have interests other than economy" which is the conclusion makes B the most appropriate.
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MGMAT 6 650 (51,31) on 31/8/11
MGMAT 1 670 (48,33) on 04/9/11
MGMAT 2 670 (47,34) on 07/9/11
MGMAT 3 680 (47,35) on 18/9/11
GMAT Prep1 680 ( 50, 31) on 10/11/11

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CR notes
http://gmatclub.com/forum/massive-collection-of-verbal-questions-sc-rc-and-cr-106195.html#p832142
http://gmatclub.com/forum/1001-ds-questions-file-106193.html#p832133
http://gmatclub.com/forum/gmat-prep-critical-reasoning-collection-106783.html
http://gmatclub.com/forum/how-to-get-6-0-awa-my-guide-64327.html
http://gmatclub.com/forum/how-to-get-6-0-awa-my-guide-64327.html?hilit=chineseburned

Mayor: I am helping the economy by building a highway
Citizens: Yes, that MAY help but a business park helps even more. Why don't you propose that? Anything we don't know?

The question asks which assumption the citizen rely on in their argument.

B) says, if the mayor knew about the business park and still chose the highway, he must have some other intention (but this answer is written in way to make the reader believe that the mayor accepts the citizens group statement, which means that he admits wrongdoing - its nonsensical)

ASSUMPTION: The mayor knows about the estimated revenue of both and still proposes the highway and therefore admits he is a crook.

E) the citizens don't believe the revenue from the highway will be enough to help the economy

ASSUMPTION: Yes, the highway will bring in revenue but not enough to help the economy, therefore the business park is the only way to help the economy.

Lets hope that none of us actually encounters anything like this, and if so, hopefully on our way to a 770.

Cheers

B only makes there argument stronger

How can choice B an assumption?? Someone please explain.

Out of all the choices, E looks okay.

Citizen's group:
Business park better than new highway.
Thus, you must have another interest in mind.

What if the mayor believes his solution is best? Then, that shows that the mayor must really have the right interest.

(B) The mayor accepts that a new business park would bring in more new business than would the new highway.

B shows that the mayor knows a better solution but chose to build a highway still...

Mayor of Plainsville: In order to help the economy of Plainsville, I am using some of our tax revenues to help bring a major highway through the town and thereby attract new business to Plainsville.

Citizens’ group: You must have interests other than our economy in mind. If you were really interested in helping our economy, you would instead allocate the revenues to building a new business park, since it would bring in twice the business that your highway would.

The argument by the citizens’ group relies on which one of the following assumptions?

The Conclusion of Citizen's group is : You must have interests other than our economy in mind.

Accordingly, the relationship between the conclusion and the assumption can be described as:
Conclusion Valid --------------> Assumption True

Hence, the answer you select as correct must contain a statement that the author relies upon and is fully committed to in the argument. Think of an assumption as the foundation of the argument, a statement that the premises and conclusion rest upon. If an answer choice contains a statement that the author might only think could be true, or if the statement contains additional information that the author is not committed to, then the answer is incorrect.

assumptions play one of two roles—the Supporter or the Defender.
The Supporter role is the traditional linking role, where an assumption connects the pieces of the argument.
Defender assumptions protect the argument by eliminating ideas that could weaken the argument

Based on this, u can know that this question is playing a defender role.The author assumes that any idea that would weaken the argument is impossible and cannot occur.

One of the approach for Defender assumption is to use the classic The Assumption Negation Technique.
1. Logically negate the answer choices under consideration
2. The negated answer choice that attacks the argument will be the correct answer.
When the correct answer choice is negated, the answer must weaken the argument. This will occur because of the conditional nature of an
assumption.

The Conclusion of Citizen's group is : You must have interests other than our economy in mind.

(A) Plainsville presently has no major highways running through it. --- Negate the statement . Plainsville presently has major highways running through it. It does not hurt the conclusion that mayor has other interest.
(B) The mayor accepts that a new business park would bring in more new business than would the new highway. --- We are asked about the citizen group assumption not what mayor accepts. And the argument does not mention any comparison on the mayor part, in which he weighs the advantage of one project over the another.
(C) The new highway would have no benefits for Plainsville other than attracting new business. ---- Correct Answer:This statement bubbles up what citizen group is been thinking. If you negate this statement that The new highway would have benefits for Plainsville other than attracting new business. Then the arguement's conclusion falls apart. What citizen group is assuming that the only benefit is to attract new business and the new highway can in no which way help to generate more revenue.
(D) The mayor is required to get approval for all tax revenue allocation plans from the city council. -- This does not defend the conclusion that the mayor is interest other than economy.
(E) Plainsville’s economy will not be helped unless a new business park of the sort envisioned by the citizens’ group is built. --- This is a classical trap. If you look closely, you will find that it uses the word "unless", but the citizen group argument above only mention about the alternative. It does not say that the only way to help economy is to build the park.