The neurotransmitter has been a primary focus of drug abuse research because of its well-known effect on a part of the brain's reward system known as the nucleus accumbens. drugs that increase the activity of this neurotransmitter are most likely to result in physical dependence.

The correct answer is:  "dopamine" .

            "The neurotransmitter    dopamine  has been a primary focus of drug abuse research because of its well-known effect on a part of the brain's reward system known as the nucleus accumbens.  Drugs that increase the activity of this neurotransmitter are most likely to result in physical dependence."

Well if you pick a machine to work on and that's gonna be your exercise if you don't stick to that machine your muscle strength your muscle mass rather will change because you haven't been doing it you change something in your exercise routine which changes your body's form of dealing with things for example you might become weaker or stronger depending on what kind of exercises you do in whether or not the machine you picked actually your muscles grow.