A study followed a group of teenagers who had never smoked and tracked whether they took up smoking and how their mental health changed. After one year, the incidence of depression among those who had taken up smoking was four times as high as it was among those who had not. Since nicotine in cigarettes changes brain chemistry, perhaps thereby affecting mood, it is likely that smoking contributes to depression in teenagers.

Which of the following, if true, most strengthens the argument?


Participants who were depressed at the start of the study were no more likely to be smokers after one year than those who were not depressed.

The study did not distinguish between participants who smoked only occasionally and those who were heavy smokers.

Few, if any, of the participants in the study were friends or relatives of other participants.

Some participants entered and emerged from a period of depression within the year of the study.

The researchers did not track use of alcohol by the teenagers.

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情景：研究发现，凡是吸烟的人都更容易抑郁，所以我们认为，吸烟会导致抑郁症的出现。

推理：“吸烟”和“抑郁”这两个事件在前提中具有统计关联，即，the incidence of depression among those who had taken up smoking was four times as high as it was among those who had not。

推理结构为：

前提：凡是“吸烟”的人都更容易“抑郁”（“吸烟”和“抑郁”之间存在正相关关系）
结论：吸烟导致抑郁

选项分析：

A选项：Correct. 相对于那些在不抑郁的人，凡是那些在研究最开始就抑郁的参与者不会更有可能成为吸烟者。本选项既谈到了“吸烟”，又谈到了“抑郁”，因此可以保留。实际上，这个选项内含的意思是：抑郁不会导致吸烟。若抑郁会导致吸烟，那么显然可以削弱推理文段，属于CQ3：因果方向问题。现在将该削弱项取非，自然可以加强推理文段。

B选项：研究没有区分哪些偶尔抽烟和重度抽烟的人。本选项只谈到了吸烟，没说到抑郁，可以排除。

C选项：很少有研究的参与者是其它参与者的朋友或者亲属。本选项完全没有谈到吸烟和抑郁，可以排除。

D选项：某些参与者在一年的研究中展现了一段时间的抑郁。本选项只谈到了抑郁，但是没有给抑郁的产生提供别的原因，因此可以排除。

E选项：研究人员没有跟踪青少年的喝酒情况。本选项完全没有谈到吸烟和抑郁，没有理由相信喝酒可以导致抑郁，因此可以排除。

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