Hi @OQ, thanks for your post. This stimulus presents an argument; the conclusion is "the soot itself probably does not cause this ailment." Why? The given premise is that "in cities where there are large amounts of soot in the air, there are usually also high concentrations of many other air pollutants."
The question stem asks you to weaken this argument. Answer choice (C) does weaken the argument; if true, it would show that in cities where there are large amounts of soot, the frequency of the ailment at issue is at least as high as it is anywhere else - even if there is little other air pollution in those cities. This fact would undermine the conclusion presented in the stimulus that "the soot itself probably does not cause this ailment."
Hope this helps! Please let us know if you have any additional questions.
