Some doctors believe that a certain drug reduces the duration of episodes of vertigo, claiming that the average duration of vertigo for people who suffer from it has decreased since the drug was introduced. However, during a recent three-month shortage of the drug, there was no significant change in the average duration of vertigo. Thus, we can conclude that the drug has no effect on the duration of vertigo. Which one of the following is an assumption required by the argument? ['Changes in diet and smoking habits are not responsible for any change in the average duration of vertigo since the introduction of the drug.', 'If there were any change in the average duration of vertigo since the introduction of the drug, it would have demonstrated that the drug has an effect on the duration of vertigo.', 'A period of time greater than three months would not have been better to use in judging whether the drug has an effect on the duration of vertigo.', 'If a drug made a difference in the duration of vertigo, a three-month shortage of that drug would have caused a significant change in the average duration of vertigo.']