PT1 · S4 · Q12 A recent study found that snoring

This just identifies a cause of smoking. So it doesn't provide an alternate explanation for why smoking / snoring are correlated, nor does it attack the storyline that smoking causes snoring. We could try to make this a Third Factor alternate explanation by saying, "Smoking and snoring are both symptoms of obesity", but this answer doesn't connect obesity and snoring. So we would need that to be a common sense link. Are snoring and obesity already linked in our common sense? Not mine. And (A) doesn't force me to draw upon hidden connections like that, so the impact of (A) is clearer.

(C) and (D) both feel like they implicitly attack the connection between snoring and smoking, but in the case of (C), was the author thinking that most snorers are smokers? No, the author's hypothesis is very weak: smoking can induce snoring. The author is only claiming it's one possible cause of snoring. There might be many other more common causes of snoring. So it doesn't hurt the argument to say that most snorers don't smoke. The author is specifically looking at the fact that smokers have a higher rate of snoring than nonsmokers and inferring that smoking is causally contributing to their snoring. As an analogy, we might say "though not common in either group, being a baseball pitcher is more common among left handed people than among right handed people. Thus, being left handed helps you be a better pitcher." This is basically a true real world fact. It's harder for hitters (most of whom are right handed) to hit a pitch thrown by a leftie. So their left handedness is actually contributing to them being a left handed pitcher. Nonetheless, most pitchers are righties. That's because there are way more righties in the world than lefties (like 8x as many). So even though being a leftie helps you be a better pitcher, that doesn't mean that most pitchers are lefties, because the underlying numbers are still so skewed toward righties. Similarly, even if smoking does make you more likely to snore, the fact that there are so many more nonsmokers than smokers in the world would still lead us to expect that most snorers are nonsmokers.

(C) and (D) both feel like they implicitly attack the connection between snoring and smoking, but in the case of (D), was the author thinking that most smokers are snorers? No, the author's hypothesis is very weak: smoking can induce snoring. And we already know from the first sentence that snoring is not common among smokers. The first sentence already tells us (D), so there's no way that repeating this fact could weaken the argument.

This is masquerading as a Third Factor alternate explanation, but for that sort of answer we want smoking / snoring to both be Effects of some common Cause. In this answer, smoking / snoring are both Causes of some common Effect. This answer doesn't give us a different way to explain why there is a correlation between smoking and snoring.