Explanation

@Boram, Great question. Let's take a look at this question to see why the answer is A. The conclusion of the argument is that the public evidently heeded the campaign, and the support given is that influenza rates when down during the six-month public health campaign. This is a classic correlative premise with a causal conclusion (hypothesis). The question asks us to select an answer that would most strengthen the argument. In these types of arguments, there are a number of ways in which we can strengthen. We can block alternative hypotheses or we could assume the hypothesis is true. If we assume the hypothesis is true, then it would make certain predictions. Then we'd look for verifications of these predictions. Let's assume that the author's conclusion/hypothesis is true: the public heeded the campaign. If this were true, then that means the public took precautions such as washing their hands frequently and avoiding public places when they had influenza symptoms. And if this were true, a prediction that would come out of this is that whatever other illness that could be prevented by frequent handwashing would also be reduced. This is what answer choice A picks up on, and it's why A is correct. A says the incidence of food-borne illness, which can be effectively controlled by frequent hand washing, was markedly lower than usual during the six-month period. If we assume that the author's hypothesis is true, then we would expect that frequent handwashing would diminish the incidence of food-borne illness since food-borne illness can be controlled by frequent handwashing. This is what A says. In doing so, it adds further support to the hypothesis that the public listened to the campaign and started washing their hands more. Answer B is incorrect because it does nothing to strengthen or weaken the argument. We have no idea whether occurrences of the common cold can be reduced by the same activities that the campaign promoted. Answer C is incorrect because it introduces an alternative cause for why the incidences of influenza reduced during the period. If this were true, it would actually weaken the argument. Answer D is incorrect because it introduces an alternative cause for the public's change in behavior. If news media spread the message that more handwashing reduced the risk of influenza, and they spread this independently of the campaign, then how do we know the public heeded the campaign? This answer also weakens the argument. Answer E is incorrect because whether people think they should do something and whether or not they actually do the action is another question. This doesn't tell us if these people actually changed their behavior, so it doesn't do anything for us. We can get rid of this answer choice. Does this make sense? Let us know if you have any more questions!