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?