A study followed a group of teenagers

[RonPurewal]

no. we're not "eliminating an alternate cause".
"eliminating an alternate cause" would mean showing that ____ doesn't cause depression, in which "____" is something other than smoking.

rather, we're eliminating the possibility that the causation runs in the opposite direction.

[chetan86]

Hi Ron,

Thanks a lot for finding my mistake.
Now I am clear about the question and answer choices.

Regards,
Chetan

[RonPurewal]

excellent.