Typically during thunderstorms most lightning strikes carry a negative electric charge; only a few carry a positive charge. Thunderstorms with unusually high proportions of positive-charge strikes tend to occur in smoky areas near forest fires. The fact that smoke carries positively charged smoke particles into the air above a fire suggests the hypothesis that the extra positive strikes occur because of the presence of such particles in the storm clouds. Which of the following, if discovered to be true, most seriously undermines the hypothesis? ['The positive-charge strikes that occur near forest fires tend to be no more powerful than positive strikes normally are.', 'Other kinds of rare lightning also occur with unusually high frequency in the vicinity of forest fires.', 'Thunderstorms that occur in drifting clouds of smoke have extra positive-charge strikes weeks after the charge of the smoke particles has dissipated.', 'A positive-charge strike is as likely to start a forest fire as a negative-charge strike is.']