Dopamine is a chemical released by neurons to send signals to other nerve cells. the brain includes several distinct dopamine pathways, one of which plays a major role in reward motivated behavior.
smoking is a habit, in a habit there is the cue, routine, and reward. with smoking the cue can be different things, most commonly stress or anxiety. the routine how you react to those cues, the reward is the chemical 'triggers' that are pulled inside you're brain leaving you feeling calm, happy, or relieved, etc.
C. From carbon dioxide in the atmosphere
I think its C... but I'm not too sure. I did some research and couldn't find exactly what you were looking for but, I found loopholes and I think since when the cell increases, the surface area does not affect cell divisions steps. Therefore, I don't think it would change any other way...
