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.
This strengthens by providing further evidence that hand-washing rates were higher during the six-month period. This makes it more likely that during this time period, the public was actually heeding the campaign, which encouraged hand-washing.
Presenting evidence that corroborates (in Strengthen) or conflicts (in Weaken) with the author's hypothesized explanation or the predictions that follow from that explanation.
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.
This might actually weaken, since the spread of the common cold would probably also be limited by the measures encouraged by the public health campaign. So if common cold rates were not lower, the campaign may not have caused the lower influenza rates.
Answers that, if they have any effect, do the opposite of what we want (weaken when we're trying to strengthen, or strengthen when we're trying to weaken).
There were fewer large public gatherings than usual during the six-month period.
It’s not clear without more information whether this is connected to the public health campaign, so it doesn’t help us. This might be due to the campaign’s advice to avoid gatherings when ill, but it could also be unrelated—we can’t jump to either assumption.
Answers that, if they have any effect, do the opposite of what we want (weaken when we're trying to strengthen, or strengthen when we're trying to weaken).
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.
This weakens by providing an alternative explanation for the reduction of influenza rates: instead of heeding the public health campaign itself, the public may have just been listening to the news media’s reports.
Answers that, if they have any effect, do the opposite of what we want (weaken when we're trying to strengthen, or strengthen when we're trying to weaken).
In a survey completed before the campaign began, many people admitted that they should do more to limit the spread of influenza.
This is irrelevant, since the public’s desire to limit the spread of influenza does not provide evidence that they followed the instructions of the public health campaign later on.