The reason D is incorrect is that the argument doesn't require that most smokers snore. In fact, it says specifically in the premise that snoring is not common among smokers or non-smokers. The author only hypothesized that smoking can cause snoring. Answer choice D does not weaken this argument. It doesn't matter that most smokers do not snore, as long as they snore more than non-smokers. This is why A is a better answer. It provides an alternative explanation, or a third factor, that leads to both smoking and snoring.
