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.