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?

A. 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.

B. The study did not distinguish between participants who smoked only occasionally and those who were heavy smokers.吸烟的严重程度不是重点

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

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

对，此题是典型的因果倒置。两个现象A , B同时出现，原文就妄下结论说是A导致B，或者B导致A；那么要重点反对原因，只需要用相反的关系就可以驳倒它。、

至于D，无关原因。而且有一个小词some,是不定选项。