A six-month public health campaign sought to limit the spread of influenza by encouraging people to take precautions such as washing their hands frequently and avoiding public places when they experience influenza symptoms. Since the incidence of influenza was much lower during those months than experts had predicted, the public evidently heeded the campaign. Which one of the following, if true, most strengthens the argument? ['The incidence of food-borne illnesses, which can be effectively controlled by frequent hand washing, was markedly lower than usual during the six-month period.', "Independently of the public health campaign, the news media spread the message that one's risk of contracting influenza can be lessened by frequent hand washing.", 'During the six-month period, the incidence of the common cold, which has many of the same symptoms as influenza, was about the same as usual.', 'There were fewer large public gatherings than usual during the six-month period.']