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题库 / GMATLA-CR-59
Researchers took a group of teenagers who had never smoked and for one year tracked whether they took up smoking and how their mental health changed. Those who began smoking within a month of the study's start were four times as likely to be depressed at the study's end than those who did not begin smoking. 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 study's start were no more likely to be smokers at the study's end than those who were not depressed.
Participants who began smoking within a month of the study's start were no more likely than those who began midway through to have quit smoking by the study's end.
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.
a的意思是说，已经抑郁的人不会更有倾向去吸烟。这个从反面证明了吸烟导致抑郁。因为如果抑郁的人会更倾向去喜欢去吸烟，那么测试结果就可能因为样本bias而出现偏差：即抑郁的人喜欢吸烟，所以吸烟人群中抑郁的人多。而排除了这种可能性使得实验更加valid。确实绕。
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