The stem is asking us to pick an answer choice that'll weaken the argument.
According to economists, money is the biggest motivating factor when someone is picking a job. However, a survey showed that other things are more important to people than salary, so the argument concludes that the economists are mistaken.
In looking at the argument, it's true that the survey takers did not state that a high salary was the single most important factor in their decision, but just because they didn't name it to be the most desirable component of a job does not prove that the economists are wrong. In addition to salary, there are all sorts of other financial benefits that may come with a job, including healthcare, free food, free transportation, etc. Put together, all of these factors could end up being the most important thing to people when looking at different jobs.
(C) says, "Jobs that pay the same salary often vary considerably in their other financial benefits."
(C) looks great, as the phrase "other financial benefits" points out that there are things other than salary that are related to financial benefits, and that these things could be very important to people. This casts doubt on the author's conclusion because the author's conclusion is only looking at salary, and this shows there are other financial considerations in addition to salary. Thus, (C) is the correct answer choice.
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