[quote2gmatjp]what does this counter mean? is it weaken?[/quote2]
Essentially, yes. "Counter" = "go against" = "weaken".
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We need a choice that counters the business spokesperson's rebuttal (that it is "basic economics to put up with the pollution".)
Choice A tells us that the town is losing money because of the snowmobiler's pollution. (Reduced visitors = lost money). So, if the town is losing money because of the pollution, this attacks the business spokesperson's claim that putting up with the pollution is economical. Thus, choice A is correct.
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[quote2dwivedys]is there any big difference between saying "not all of the people" and "a great many cross country skiers..."[/quote2]
There is a difference between "a great many" and "not all". "a great many" implies an appreciable and significant quantity. But with "not all", a speaker is conveying something else--trying to draw attention away from that quality. If I say "a great many Torontonians are nice", clearly my intention is to say something good about Torontonians. Now, if I say "not all Torontonians are nice", clearly my intention is different. In the case of choice B, "not all" can mean as few as "one", or up to 99%. If it was just one, there is little impact on the argument while if it is 99%, there is a much bigger impact on the argument. Because we don't know which, we can't evaluate what impact choice B has on the argument and, therefore, cannot conclude that choice B weakens.
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Choice D discusses "industrial pollution" which is, likely, different from air pollution due to snowmobiles. At any rate, choice D discusses reducing pollution. But to counter the business spokesperson's claim, we needed a choice that said that pollution (i.e., NOT reducing pollution) isn't economical.
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[quote2dwivedys]isn't cross country skiers a very specific set of people[/quote2]
The stimulus tells us it's the kind of town in which "snowmobiling" is a big activity in the winter. The spokesperson discusses money collected from "recreational use of the park". It is reasonable to infer that recreational winter activities at the park, if they include snowmobiling, can also include "cross-country skiing".
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@dwivedys: essentially, I think you converted "counter spokesperson's claim" to "strengthen first speaker's claim". Can't quite do that here because the businessperson is responding to the first speaker by making a claim of his own--a claim, which, according to the instructions in the question stem, we have to directly counter using one of the answer choices...always follow the instructions in the question explicitly!