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? 



(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.


Let’s look at the details of the argument


Premises


1)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. 

2) 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.

3) nicotine in cigarettes changes brain chemistry, perhaps thereby affecting mood


Conclusion-

It is likely that smoking contributes to depression in teenagers.


We need to find an option that strengthens the conclusion.


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.


Let’s try and negate this- participants who were depressed at the start of the study were more likely to be smokers at the study's end than those who were not depressed. This reverses the cause and effect sequence in the argument. This would mean that they took to smoking because of depression and not vice versa. This would seriously weaken the conclusion that smoking contributes to depression.  

Option A says that people who were depressed and who were not depressed at the start of the study were equally likely to start smoking. This supports the conclusion that smoking contributes to depression. Hence A is correct.


(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.


The comparison is only between the mental state of the participants who began smoking and that of participants who did not. This is not relevant to the conclusion of the argument. Eliminate B


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


This is totally irrelevant to the conclusion- It is likely that smoking contributes to depression in teenagers- Eliminate C


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


Emerging from depression is not relevant to the conclusion. Eliminate D


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


Out of scope. The argument only talks about the relation between smoking and depression. Eliminate E