A positive correlation has been found between the amount of soot in the atmosphere of cities and the frequency of a certain ailment among those cities' populations. However, the soot itself probably does not cause this ailment, since in cities where there are large amounts of soot in the air, there are usually also high concentrations of many other air pollutants. Which one of the following statements, if true, most weakens the argument? ["If high concentrations of many different pollutants in a city's air are correlated with a high frequency of the ailment among that city's population, then it is possible that two or more of those pollutants each causally contributes to the ailment.", 'In cities in which there are high concentrations of many air pollutants, there are generally also high concentrations of other forms of pollution that are very likely to contribute causally to the ailment.', 'If the ailment rarely occurs except in cities in which there are large amounts of soot in the air, then the soot is probably the cause of the ailment.', 'In each of the cities where there are large amounts of soot in the air but little other air pollution, the frequency of the ailment is at least as high as it is anywhere else.']