OFFICIAL EXPLANATION

(A).

This is a Weaken the Conclusion question. The correct answer choice will cast doubt on the sponsors' claim that "reducing nicotine intake does not improve one's health."
(A) CORRECT. This answer choice indicates that most study participants did not actually reduce their overall nicotine intake; instead, they replaced the reduction in cigarette-based nicotine with nicotine from other sources.
(B) Although this choice does indicate that a number of study participants might have consumed nicotine from secondhand smoke, it does not undermine the idea that these individuals, by cutting back their smoking, have significantly reduced their nicotine intake. Before the study, the nicotine intake of these participants was smoking plus secondhand smoke; now, the only nicotine intake is secondhand smoke.
(C) This choice neither strengthens nor weakens the conclusion. The study only addresses the reduction of nicotine intake through smoking and never mentions these other factors. That the study used a diverse sample of smokers does not, in itself, undermine the validity of the conclusions.
(D) This answer choice is irrelevant. The study concerns the effects of reducing nicotine intake through smoking, not quitting smoking entirely.
(E) This choice is irrelevant. While it further details the methodology used in selecting subjects, it neither strengthens nor weakens the conclusion.