This strengthens the author’s argument. Since the ailment also occurs in places where there’s little to no soot in the air, it seems more likely that other pollutants, not soot, are causing the ailment.

It definitely seems true that if the ailment mainly showed up only in cities with high levels of soot in the air, then soot would be the likely cause. But We don't know if the \"if\" part of this statement ever occurs. Since this is a hypothetical statement, it can’t weaken the argument.

This weakens the author's argument. The ailment is just as common when there is only soot in the air, and not many other pollutants. This makes soot seem like the likely cause of the ailment, contrary to the author's conclusion.

Notice that, while this answer choice leaves open the possibility that soot is a contributing factor to the ailment, it also leaves open the possibility that it's several of the other pollutants, but not the soot, that cause the ailment. So on its own, this answer choice doesn't give us a reason either to favor or discard the author's conclusion. It doesn't make soot more or less likely to be a cause of the ailment. So this doesn't weaken the conclusion.

While this answer choice technically leaves open the possibility that soot causes the ailment, it could work just as well as a neutral description of the city atmospheres described by the author in the stimulus. This answer choice on its own doesn't make soot more or less likely to be the cause of the ailment, so it doesn't weaken the conclusion.