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? ['Few, if any, of the participants in the study were friends or relatives of other participants.', '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.', 'Some participants entered and emerged from a period of depression within the year of the study.', 'The study did not distinguish between participants who smoked only occasionally and those who were heavy smokers.']