This does not point out any unaddressed effects of the tax that might keep it from reducing smoking. Instead, it presents a condition which, if met, would indeed reduce the amount of smoking in the city.

There may be better ways to reduce cigarette purchases, but (B) doesn't weaken the argument that taxes would have some impact. Also, we still don’t know if a reduction in purchases would reduce smoking, or if the tax has some other effect that would prevent a decrease in smoking.

(C) weakens the author's argument by pointing out a potential effect of the tax that might keep it from reducing smoking. Since people could find other ways to get cigarettes after the tax increase, a reduction in purchases might not mean less smoking, as the author assumed.

This implies that the antismoking education programs could reduce smoking. But it doesn’t undermine the author’s conclusion, which is that increased cigarette taxes would reduce smoking. We need an answer that addresses the effects of the tax, not of the education programs.

Like (D), this speaks to the effectiveness of antismoking education programs; if they successfully reduce smoking, then they’ll lose their funding due to fewer cigarette purchases. However, it doesn't weaken the author's conclusion that higher cigarette taxes will reduce smoking.