GWD-9-Q37，请教背后的逻辑关系

GWD-9-Q37:

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.

A.      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.

B.       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.

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

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

E.       The researchers did not track use of alcohol by the teenagers.

答案A，虽然BCDE都明显不对，但还是想请教大家，A答案背后的逻辑是什么？

A是告知 學期前 沮喪的學生跟不會沮喪的學生學會抽菸的機會是一樣多的 就是說 他們的比較基礎是相當的 於是支持了 題目中 尼古丁造成的沮喪

個人愚見 請指教

如果如上所述，学期前就有学生沮丧，就不能证明是由于尼古丁影响了情绪，而是学生本身就有沮丧的情绪了。。。

这个怎么解释呢？