Letter D would be likely it
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.
D) each parent passes only one factor for each trait to its offspring
The testicles produce the sperm and the hormones
